The Development of Low-Floor Trams

Transcription

1 Journal of Advanced Transportation, Vol. 27, No. 1, pp The Development of Low-Floor Trams Introduction Harry Hondius The gradual development of trams, i.e., streetcars or trolley cars, from 1950 till 1980 was abruptly interrupted in the early eighties by the arrival of the low-floor car. Six factories in FRG and Austria, one in Switzerland and one in Belgium virtually covered the market based on what roughly could be defined as three designs. Today each of these makers has its own design(s) complemented by those of four italian, one swiss and one french builder. This article aims to give a synthesis of the development. Which designs have so far emerged as leading in volume, what are the technological tendencies that can be traced. What are the price consequences of the development. This article deals with trams, i.e., streetcars or rail vehicles, with a width not larger than 2.4 m that can be directly boarded from street level and have a practical max. speed not exceeding km/h. It does not deal with LRV s (Rapid Transitcars) like the 2.65 m wide U2 highfloor cars that are running in Calgary and Edmonton etc. The tram fleet in the Western world (Yugoslavia and Japan excepted) amount to approx. 17,000 vehicles. Until the beginning of the eighties, since Austria, Benelux, Germany (FRG) and Switzerland had the largest fleets, to a certain extent standardized vehicles like Stadtbahn M and N- cars were produced by leader DUEWAG(FRG) and its licensees, Tram 2000 modules by Schindler/SIG (Switzerland) and BN (Belgium) largely based itself on the PCC-bogie-technology. Floor heights were at 880mm, generally accessible by three steps ( x 185 mm). The DC-motors, with the exception of those mounted in the BN-cars, were all of the monomotor type, mostly chopper-controlled with recuperation of the braking-current. Wheel diameters were mm and the specific weight had crept up from 450 kg/m2 in the sixties to kg/m2. This state of the art, which was the result of 30 years gradual development following WWII, was drastically shaken by the introduction of alow-floor tram by Transports Publics GCnCvois (TPG), Geneva in Since then the world of tram suppliers has been turned upside down. Around 15 July cars withvariousdegreesoflow-floor percentages have been ordered or are already delivered, 26 designs by 15 suppliers have been announced and the end is not yet in sight. This article aims to inform about trends in design and technology of anumber of examples of series of greater importance in the world market. Harry Hondius, Chaudfontaine, Belgium.

2 80 Harry Hondius The advantages of a low-floor with a height of about 350 mm above the railhead are: - easier access to the tram by all passengers specially by pramusers. - quicker boarding/alighting of passengers at stops. - the fact that, if the height of the stops is brought to the floor level, wheelchair-users may roll on board. 1. Category. Conventional 8-axle cars with low-floor center sections mostly at 10% but occasionally up to 48% of the car length, mostly with floating articulations. A number of new cars, length between 26 and 33 m, were ordered orexisting six-axle trams were equipped with acenter section which has its entrance part at mm (Fig. 1). This method also offers the possibility to have all axles motored. 108 new cars have so been ordered. Half of the new cars are equipped with AC-motors which drive the axles individually. 82 cars have been converted. The powerfweight (p/w) - ratio varies between kw/t, the spec. weight (sw) from kg/m2. 2. Category. Cars with classic motor bogies and special carrying bogies or wheelsets to enable a low-floor over 50-73% of the length of the cars, which varies from m. Wheel arrangement B 2 B, B 2 2 B or Bo 2 2 Bo. 2.1 VeveY/DUEWAG - designs based on the DUEWAG M6 and M8-designs but now with small-wheeled (diam. 4 10mm) carrying bogies and floating articulations. (Fig. 2 and 3). The driving wheel diameter is 560 mm, the high-floor section is situated at 710 mm, two steps interlink both sections. 71 six-axle and 12 eight-axle cars of this meter gauge type are in service in Geneva, Berne and St. Etienne (F). LHBWaggonbau Dessau received orders for 120 similar normal gauge cars with 60% of the floor at 350 mm. p/w: 9-10 kw/t, sw: kg/ m2. DC-monomotors and chopper control were maintained but the LHB-cars have individual AC-motors. The high-floor level is at 570 mm, allowing one step between high- and low-floor part. The riding qualities, noise-level and wear and tear on the small-wheel bogies have so far given satisfaction. Very fine tuned electronics are necessary to avoid overbraking of the small-wheelset. The same equally applies to the next categories. This category totals 202 cars.

3 The Development of Low-Floor Trams 81 Figure 1. N8S Stadtbahnwagen built by DUEWAG/MAN/Siemens in The 9% center low-floor section is being added in mlong,2.3mwide,32.8t,2x120kW, 51 seats. Floorheight 880mm in the high-floor and 284 mm in the low-floor section. (DUEWAG).

4 + Figure 2. VeVeY/DUEWAG/ABB, 73% low-floor car for Bern, in service since 12/1990. Length 31 m, width m, weight 34 t, 2x15 1 kw, 68 seats. high-floor at 710 mm, low-floor at 350 mm with intermediate entrance step at 160 mm. DUEWAG DC-monomotor bogies, weight (VeVeY). $ F

5 Figure 3. VeVeY bogie for the Bern cars, wheel-base 1000mm, Bochum 84 resilient wheels, dim. 410 mm. The axle boxes are linked to the frame. Primary springs are Pierelli rubber elements, side links connect the bolster to the frame and via a ball bearing crown with the car body. The steel secundary springs are seconded by vertical shockabsorbers. A horizontal shockabsorber dampens the movement between carbody and bolster. Hanning & Kahl electro-hydraulic spring loaded disbrakes. Weight 1950 kg.

6 a4 Harry Hondius 2.2. DUEWAG (93 cars), GEC-Alsthom/de Dietrich (75) (Fig. 4), Fiat/Firema (54 for Torino) and Socimi (34 for Roma)-designs. (Wheel arrangement: Fiat and Socimi Bo 2 Bo ). Here the center section is in fact a small car, carrying the articulations, fixed to a double wheelset (Fig. 5) with independent wheels. Except for the DUEWAG-cars all others are built for normal gauge operation, have a pw of kw/t and are heavy with a sw of kgjm2. They have DC-equipment. The meter gauge DUEWAG-cars for Valencia are 12 wheelers and the 69 for MannheimLudwigshafen are 16 wheelers, 2.4 m wide. Here the module concept is applied. The specific weights (sw) range from kg/m2. The Grenoble-type cars, of which 75 have been ordered by Grenoble, Paris and Rouen, which are infact a sort of french standard car in service since 1987, are the most publicized of those cars. The reason for this was the creation of a totally new low-floor tram system, well adapted to the old city, with all platforms at k 310 mm. The high-floors are at mm. Two steps interconnect the high- and low-floor areas. The meter gauge versions have a considerably lower high-floor level of 560 mm, made possible by the use of AC-individual traction motors and a return to the kingpin for transmission of the traction forces between bogie and car; one step between the high- and low-floor area suffices. With 283 cars, this is so far the largest group. As it allows modular enlarging of the cars, this concept may find other followers. 2.3 DUEWAG-cars with EEF (Einzelrad-Einzel-Fahrwerke), self steering individual wheel sets. In 1990 Kassel tookthe first of m cars (Fig. 6) in service. They are still equipped with DC-choppers and monomotors, identical to the N8-cars, albeit with 560 mm wheels but with two EEF-wheelsets under the center section which has floating articulations. Wheel arrangement:b l l B. Thehigh-floorareaisat700mm. Pw 11.9kW/ t, sw 456 kg/m2. Spectacular and totally new are the EEF-selfsteering wheel sets which are developed by the VDV-(the German public transportation operators) and the German rolling stock builders, supported by the federal authorities and based on patents from Prof. F. Frederich of the Aachen TU. (Fig. 7 and 8 a,b). The load on the EEFwheels is of course twice as large as the load on the original wheels of the carrying bogies they replaced. Weight saving is the biggest advantage oftheeef-sets, as is clearly reflected in the sw -number. For the first time in the history of the streetcar a single set of wheels is selfsteered. After a lot of refining the wear and tear figures in Kassel correspond now to the values of the motored bogies. What were the points that needed attention?: the industry had to reappraise again the value of primary springs!

7 The Development of Low-Floor Trams I i

8 86 Harry Hondius Figure 5. Grenoble double wheelset supporting the central articulation and firmly connected to it. Wheelbase 1900 mm, wheel dia. 66CTmm. Bochum 84 wheels, Chevron primary - and elastomer secundary springs, vertical shockabsorbers between frame and car body, weight 3950 kg. SAB electro-hydraulic brake with Faiveley anti-skid system. (Alsthom).

9 ' I 9100 p Ma8 a = 913 Ma8 b = 774 Ma8 c = 455 KL = Klappsltz Figure 6. Kassel6NG DUEWAG/AEG/Siemens 70% low-floor tram with EEF-wheelsets. Weight 30.2 t, 2x180 kw, high-floor at 700mm, low-floor at 350 mm. All wheels diam. 560 mm. (KVG).

10 Figure 7. EEF-carrying wheelset. The stubs around which the wheel can turn 15 are connected to an axle-bridge, which is itself connected by two links to the frame, which rests via four rubber primary springs on the axle bridge. Three firm links between the car body and the frame form two parallelograms which locate the frame both vertically and horizontally, as well as transmitting braking forces. A horizontal damper between the frame and the car body dampens the lateral movement. Long steel springs in series with rubber springs form the secondary flexicoil suspension, paralleled by two hydraulic shock absorbers. The BSI disc brakes, located outside the wheels, are designed in such a way as to prevent torsional moments effecting the self-steering action. A spurrod with hydraulic damper assures a parallel movement of both wheels. Weight: & 1.3. (DUEWAG).

11 The Development of Low-Floor Trams 89 n Ansicht X 4any Radaufstands - Mitte Schwenk- 1 punkt 1 achse Figure 8a. The basic principle of the EEF-selfsteering concept. The steering force Fy is derived from the wheel load Fr and the hollow wheel profile with increasing conicity which creates the angle y. Fy = Fr. tang.y. and always works perpendicularly to the rail. Pictures b and c show the wheelset from above. prof. Fredench s invention is the creation of the turning point S at a distance rl from the touching point wheeyrail. Does the wheel not run in parallel with the rail, the moment Fy x the distance from the horizontal line through S creates the correcting moment. By entering into curves the wheel will follow the rail and self-steers itself radially. (BSI). Figure 8b. The EEF-selfsteering principle applied in various positions. (BSI).

12 90 Harry Hondius - as the mass of the EEF-wheels, as with all independent wheels, is quite small and those wheel sets are only braked under emergency conditions, the fastest acting anti-skid system is necessary to avoid flat spots and thus rumbling effects. - the individual axle stubs need to be strictly parallel, demanding a high precision manufacture of the frame and of the fixations of the stub bearings. - the verticality of the position of the wheels needs to be assured under varying load conditions, imposing great precision on the steering link and damper functions. Kassel increased the number of cars ordered to 25, Bochum ordered 42 and Halle 2 bi-directional cars for meter gauge, followed by Heidelberg with 9 cars. Rostock ordered 50 single directional cars for normal gauge. AC motors made a lower height of the high-floor part possible (Fig. 9), with only one step of 2 10 mm in the Bochum case and a rampcovering mm in the Rostockone. In Kassel and Rostock tickets can be bought at the front entrance. Bonn ordered 24 nearly identical cars, which do, however, not employ EEF-sets but wheel sets with independent wheels, steered by the floating articulations. The width of all these cars is 2300 mm, the wheel arrangement Bo l 1 Bo. This category comprises 119 cars. The main tendencies of this development are: - AC motors, which make a 160 mm lower high-floor part possible, easingthe access inthe front door, should tickets be sold by the driver. EEF-wheel sets allowing a sw of 450 kg/m2. a large number of 1300 mm doors in the low-floor area. one step or ramp between the 350 mm and 540 or 560 mm zone. the low-floor entrance part is often at 300 mm. the chairs are as far as possible directly mounted on the floor to avoid putting them on boxes concealing equipment or wheels. the high-floor cross section offers often four seats in order to invite passengers, which travel over longer distances, to install themselves there. 2.4 Breda s 80% low-floor modulated and articulated design. Wheel arrangement B l l l B. Breda of Italy is testing one 22 m prototype car in normal gauge in Roma and received orders for 24 bi-directional meter gauge cars from Lille, with a length of 28 m and width of 2400 mm. This aluminum

13 The Development of Low-Floor Trams 91! Y Figure 9. DUEWAG bogie for AC-drives and 560 mm wheels. Megi-springs as primary suspension. The bolster is through two sidelinks connected to the bogie frame. Forces to the car body are transmitted by the kingpin. The secondary suspension consists of steel springs embedded in rubber and paralled by hydraulic shockabsorbers. The Siemens 90 kw AC-motors, the Flender-gearbox and the brake disc are suspended from the bogie frame. A Flender cardan coupling drives the axles. Wheelbase 1800 mm. (DUEWAG).

14 92 Harry Hondius design consists of two 100% low-floor passenger modules of 4.8 m each, resting on single wheel sets, diameter 500 mm, (Fig. 10) mounted under the articulations and two driving modules of which the 100% lowfloor part occupies 5.25 m each. A transverse-mounted AC motor drives both axles (wheel diameter: 680 mm) of the end bogies. 20% of the car s length is used by the driver- and traction equipment compartment, situated at 950 mm. pw: 12.3 kw/t, sw: 480 kg/m2. 3. All-low-floor cars on newly developed running gear based on independent wheels The BremenlMAN-type. The first 100% low-floor, 12 wheel, 26.5 m. 2.3 m wide, 26.8 t design, wheel arrangement (la) (Al) (Al), was introduced in February 1990 by Bremen and was developed by that company together with MAN-GHH (now AEG-Westinghouse) and Kiepe (now GEC-Alsthom). Three mechanically nearly identical cars were delivered in 1990/91 to Munich with Siemens and AEG equipment. A meter gauge version with Siemens equipment is under construction for Augsburg. After an accumulated 100,OOO km of service running by those prototypes today the following orders have been placed: Bremen 78 cars of 35.4 m length andfourbogies, Munich70cars with three bogies. For 1000 mm gauge: 1 for Halle, 12 for Zwickau and 12 for Braunschwerf s mm gauge all with three bogies. Totally 178 firm orders, with option agreements signed for a further 65 cars. This design today leads the field. It is based on the long standing practice which Bremen (and for that matter Munich) had with 16 m long single articulated trams with bogies placed under the center of each car part. A three part double-articulated version was created in 1986 and thoroughly tested. It showed that the bolster springs could perfectly guide the double articulated cars in curves and still give a stable run on straight track. The adopted design is modular, it can be delivered as a two-, - three- or four bogie car (Fig. 11). Each car section rests on its own bogie (Fig. 13). The AC-motor is hung to the car floor and thus fully-sprung. It drives the wheel set nearest to it through a cardanshaft. The nearest independent wheel is directly driven through a double reduction gearing. The same gear on the other side through a torsion-stiff shaft. In fact the driven independent wheel set is technically reacting as an axle, which has evident advantages regarding running stability. The other wheel set consists of non-braked independent wheels. Only 130 kg per gearbox are unsprung. 65% of the car weight rests on the driven wheel set. The rubber bolster springs make the running through 18 m curves possible. No bars or steering elements are necessary. The Munich cars have airsprings performing the same

15 Figure 10. The Breda Talgo-like suspension under the articulations of the passenger modules. Resilient wheels, no primary springs, secondary spring paralleled by hydraulic shockabsorbers. Links to the two carbodies and the ball bearing crown. Weight only 1.5 t. (Breda). \o w

16 Figure 11. BrememANGTN8 100% low-floortriple articulated car. The two middle modules form a four-axle car, its stainless steel bodies can move in relation to each other in the horizontal plane. The end cars are hinged to the center cars and can make movements in relation to the center group in the horizontal and vertical plane. 4 GEC- Alsthom AC motors of 85 kw and Kiepe direct pulse inverters. 89 seats, 34 t. (AEG)..... / X 2 E, : Y

17 The Development of Low-Floor Trams 95 function. The floor height of the ordered Bremen cars at all entrances is 300 mm, 360 mm in the car. The wheel diameter 650 mm. The Bremen and Munich cars will be equipped with a MBB-wheelchair lift (Fig. 12). Summarizing, the main characteristics are: - all seats are placed in such a way that the passengers feet do not have to leave the floor, - per module one air-cooled, fully sprung AC-motor is necessary. - one air-cooled GTO direct puls inverter per two motors. - very low unsprung weight. Figure 12. Brememiinchen MAN 100% low-floor car. Although the floorheigth at the entrance is only 300 mm, political pressure demanded installation of a MBB electro-hydraulic wheelchair lift. (H. Hondius) Other ZOO% low-floor designs. In this category of prototypes, or relatively small orders, only the

18 I I Figure 13. BrememAN bogie. The wheelsets are suspended from the bogie frame by double linkages through Q 2 rubber bushings, allowing a certain lateral flexibility when entering into curves. Primary rubber-disc springs. Secondary elastomer springs, paralleled by hydraulic shocks. The traction forces between carbody and bogie frame are transmitted by amoment free ball linkage system. (not shown). The disc brake is placed on the motoraxle. A horizontal shock absorber between frame and carbody dampens the lateral movement. Bogie weight: 2.6 t. (MAN). 3 5 cl tz

19 The Development of Low-Floor Trams 97 Firema (I) prototype (car 5500 in Torino) has fully sprung motors connected to the bogie frame. All others have their AC-motors, mostly water-cooled, either connected to the link which guides the independent wheels (Socimi) or use hub motors with planetary gears, neither of them protected by primary springs but only by the very limited spring capacity of resilient wheels. The Socimi modular, multi-articulation, aluminum prototype for Roma as well as the subsequently 26 ordered cars for Strasbourg wheel arrangement BoBoBo2, length 32.5 m, 2.4m width, pw: 10.4 kw/kg, sw announced: 375 kg/m2, consists of wheel set -modules and passenger modules hinged between them. Three motored wheel set - modules and one none motored give as much as 6 articulations. ABBwater-cooled hub motors (up to 6000 rev/min.) and transistor inverters (one per motor) will be used. DUEWAG/Siemens supply 20 cars of very similar dimensions (width 2.35 m) and lay-out as the Bremen prototype car, but with the bogies under the A and C sections driven by water cooled Siemens hub motors, wheel arrangement Bo 2 Bo, wheel diameter of the driven wheels 740 mm, of the carrying wheels 590 mm, pw: 15.1 sw: 450 kg/ m2. Siemens IGBT (Insulated Gate Bipolar Transistor) inverters will be used, one per two motors per side. The max. revs. of the hub motors are f 4600/min. The VDV-2000 Stadtbahn prototypes produced by German industry with support of the VDV and 50% financial support of the federal state were introduced in Two m cars, one built in steel, the other in aluminum, 2.4 m wide and one m car for meter gauge, 2.3 m wide. Pw: & 12 kw/t, sw: kg/m2. The wheel arrangement being A A l or A A A l. All wheel sets are EEF and equipped with no other primary suspension than an resilient wheel. It unhappily soon became apparent that the noise level was definitively too high and the comfort level of the driven wheel sets too low, to envisage starting series production of these vehicles. DUEWAG/Siemens will in the course of the coming 12 months modify one car. It will very likely be equipped with a full primary suspension. 33. All low-floor design for Brussek This design (Fig. 14) of which 51 cars have been ordered with ACEC-Transport, a 100% GEC- Alsthom subsidiary, the mechanical parts being mainly manufactured by the BN-Division of Bombardier Eurorail, combines elements of the category 2.2 and 3.2. These bidirectional aluminum vehicles, length 22.8 m, width 2.3 m, wheel arrangement (A l )Bo( l A ), are heavy, pw: 1 lkw/t, sw: 608 kg/m2. Eight water cooled GEC-Alsthom 44 kw AC hub motors drive the three

20 Figure 14. Brussels GEC-Alsthom/Bombardier-BN, 100% low-floor car, aluminium body, 8x44kW, seating 32, weight t. (ACEC). 2;

21 The Development of Low-Floor Trams 99 double wheel sets; those placed at the outer ends of the car are of an articulated design, with some resemblance of the maximum traction truck (Fig. 15). The small independent wheel set (375 mm) steers the hubmotor driven independent wheels (640 mm) in curves. The center articulated car (cat. 2.2) is connected to a wheel set in which 4 hubmotors drive the independent wheels (Fig. 16). As with the Bremen sets, the adhesion factor-here 82%, necessary because of Brussels steep hills- does not vary with the loading. Direct air-cooled ACEC-GTOinverters feed the motors I and 4 rightside, 2 and 3 rightside and similarly the motors on the left side. The articulated double wheel sets have been tested for about 20,000 km in BN s LRV 2000 test vehicle mainly at BN s Bruges wooden sleeper test-track. Riding comfort and noise levels are perfectly acceptable. Two wheel sets will be tested during 1992/93 in an Amsterdam articulated car of category 1, replacingb3-bogies. First indications are that the wheelsets place themselves perfectly radially in curves. In BN s workshops a special rig is submitting the articulated functioning of a wheel set to a grueling continuous test. How can one summarize the development sketched under 3.2 and 3.3: a return to the beginning of electric traction: electric motors and gears are (now with only the resilient wheel protecting them) unsprung, submitted to the high vertical accelerations that tram operation, running over crossings etc., is known to produce. Values up to log are no exception. This at the same time that the Tatra, PCC-B3-similar trucks, which have the motors fully sprung, but have no primary springs are equipped by the German industry with megi-primary springs.... As DUEWAG/Siemens, Socimi/ABB and Bombardier/GEC- Alsthom are closely associated with this development, one should suppose that in practice these arrangements will work. What wear and tear they will cause to the tracks will probably remain an unknown factor. the water-cooled ABB-motors run at 6000 revsjmin. at full speed. The other makes are limiting themselves to approx in all cases the inverters will have to be able to dose the current to the AC motors pro side and bogie or double wheel set very precisely, so that the motors per side always have the same couple, preventing them to draw the wheel set on one side, with as negative consequences extra wear and tear on flanges and track. for all 100% low-floor designs, one must realize, that the possibilities for acoustic measures are limited, due to the

22 Harry Hondius Figure 15. Brussels 100% low-floor car, Bombardier-BN outer articulated double wheelsets, wheelbase 1700 mm. Swan-neck frame A is firmly connected by links (2) to the car body. The secondary suspension is by Sumride air springs (1). paralleled by Koni-shocks. A rests on subframes B through two pivots (3), which allow the B frames to pivot in relation to A. One of the pivots (with SKF-bearings) allows a slight vertical movement in relation to A thanks to (4). The wheel-andhub motor assembly comprises a resilient wheel (640 mm), a planetary gear, a water-cooled AC motor (1 2) and a pneumatic-hydraulic brake disc (1 1). This assembly is connected to B using three brackets, which form part of the motor casing, by bolt (6) bonded in rubber. Between the B frames and (6) the chevron spring package (5) acts as primary suspension. The B frames are connected to the C frames by rubber bonded pivots. C rests on the pony wheels of 375 mm (8) through rubber springs (7). B and C and (8) are interconnected by spur rods (9) to archieve radial wheel attitudes in curves and parallel operation when running on straight track. Shocks (13) dampen the lateral movement of the car body. Frames A, B and C are cast steel. Pivots are arranged in the vertical plane of the wheels so that no torsion moments are introduced into frames B or C. Weight 2.8 t. (BN).

23 The Development of Low-Floor Trams 101 Figure 16. Brussels 100% low-floor tram, Bombardier-BN double wheelset placed under the central articulation-car. The hub motor assembly, the primary suspension, disc-brake and track-brake are identical to the set described in Fig. 15. The welded frame is linked to the car body, the air springs are paralleled by shocks and a horizontal shockabsorber dampens the body s lateral movement. Rubber bump stops limit the side movement as in Fig. 15. Weight 3.8 t. (BN). - - limited vertical space available. Clients would be well advised to employ experienced acoustic consultants in an early stage of the design. There is no reason, why a modem tram should be more noisy than amiddle class car at the same speed. direct inverters may inspire AC motors to be unpleasantly noisy when starting. under seat boxing is out Other prototypes. SGP/Elin are testing prototype elements of a 100% low-floor tram with the entrances situated only at 157 mm and the floor height at 197 mm. The AC motors are mounted vertically in the articulations. SIG/Schindler/ABB are working on a meter gauge prototype with

24 102 Harry Hondius all eight wheels (for a 24.9 m car) motored and steered by the articulations. This is based on a four-axle articulated tram delivered in 1966 to Bern which had the single axle trucks successfully steered by the articulations. The motors will be fully suspended from the body frame. As the designs 3.2 to 3.4 will start running in 1993/94, only from 1995/96 onwards will one be able to judge their performance, wear and tear and noise levels That will be the moment, that a more complete judgment of this development will be possible. 4. The price development. The price consequences of this development are as dramatic as the design changes. The price of a German low-floor bus (1 1.8 m x 2.5 m) is around 13,400 DM/m2,1988/89 tram prices were around DM 33,000/ m2, at that time 2.7 times the price of a standard low-floor bus. Now low-floor tram prices range from 42,700 till 60,000 DM/m2 or between 3.2 and 4.5 times the price of a bus/m2. Even if the tram has a write-off period which is generally twice as long as the bus (25 years), this makes the tram comparatively expensive and its role should therefore really be confined to systems which carry a considerable passenger load, say not below 12,000-16,000 passengers/direction/day/line i.0. to arrive at a reasonably acceptable cost/passenger km. This point should be seriously weighed when considering the well known specific advantages of electrically driven, guided public transportation systems. Literature in Railway Gazette International, 3/1988, R. Alfter, A. Muller-Hellmann, Low floor and no axles. 5/1989, From low-floor tram to low-floor train. (VeVeY development) *5/1990, Europe refines the low-floor tram (Bremen, Bern) *6/1990, BN boards low-floor bandwagon. *1/1991, Stadtbahn 2000 to roll in the spring. *6/1991, VDV protypes reach the testing stage. *11/91, Low-floor development out of control, (5 pages + a summary table). * Stadtverkehr 3 and 4/1992: OPNV-Niederflur-Fahrzeuge im Kommen (3) (24 pages) in Urban Transport International, 11/12,1990, Hub motors for BN s ultra low floor LRV /10, 1991, Axle-less bogies open door to low-floor LRV. (Socimi) *articles published by the author.