Model Railway Reverse Loops How to automate reversing tracks on your two-rail layout, whether DC, DCC or AC powered

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1 Model Railway Reverse Loops How to automate reversing tracks on your two-rail layout, whether DC, DCC or AC powered Because of the way power is supplied to electric model trains, two-rail layouts require special treatment of reversing tracks. Wiring reversing tracks (reverse loops or wyes) is a mystery to some modelers, so they avoid using them on their layouts and miss out on the benefits of having a reverse loop or wye track. If reverse loops and wyes are controlled by manual switches, the train operator must flip them in proper sequence. A moment of inattention results in a short circuit or derailment. This is especially likely (and embarrassing) when operating the layout for visitors. On this page we briefly explain the theory of two-rail reverse loop operation, then show you how to completely automate them. Enjoy the benefits of reverse loops without the operational headaches. The Why and How of reverse loop wiring The first diagram shows a train (green arrow) entering a two-rail reverse loop. The insulated rail joints are necessary because without them, electricity would flow from one side of the power pack, around the loop and directly back to the other side of the power pack. This would be a short circuit and would prevent the train from getting power. It would also pop the circuit breaker in the power pack. The reason for two sets of insulated rail joints and the two sets of electrical connections to the power pack will be shown in the next two figures. The train travels around the loop and is approaching the exit, ready to rejoin the main line. But there will be a problem when the train attempts to cross the insulated rail joints where the loop meets the main line. The metal wheels of the locomotive will bridge the insulated joints, creating short circuits. Electricity from one side of the power pack will be able to return directly to the other side of the power pack. The train will not receive power, and the circuit breaker in the power pack will trip.

2 The solution is to swap the power pack connections at the main track before the train leaves the loop. This is done with a DPDT (double pole, double throw) toggle switch or relay as shown in the third figure. For layouts that are powered by DCC or AC power supplies, we have the option of swapping the power connections to the loop instead of the main line. All that matters is that the power connections match at the insulated joints where the train is ready to cross. On layouts with DC power, the track power connections must be swapped on the track that is not occupied by the train. On DC layouts, the track polarity (which rail is positive vs. negative) determines the direction of train travel. If we changed the polarity of the loop power while the train is in the loop, the train would suddenly change direction. Therefor we must change the main track polarity while the train is on the loop track. As you can see, every time a train travels around the reversing loop, the track polarity must be changed and the track switch must be set. If left to manual control, these are two opportunities for error.,

3 Model Railway Single Reverse Loop Wiring How to wire a single reverse loop on your two-rail layout for automatic operation Two-rail model train layouts require special treatment of reverse loops. Wiring a reversing loop (or turn-around track) may seem complicated, but if you take one step at a time and check your work, you will have a fully automated reverse loop and worry-free train operation. Here is a plan for a single 'tear drop' loop: This system is completely automated. Trains alternate their direction of travel each time they go around the loop. A slow-motion switch machine (Tortoise or SwitchMaster ) is powered by twin DC power supplies. This system, shown at right, uses a DPDT relay to control polarity of the main line. (A similar system for use with a solenoid operated, or 'snap' switch machine, is shown below.) When the relay is 'off' the main line polarity will match the loop polarity at point 'M', allowing trains to cross there. When the relay is 'on' the main line polarity will match the loop polarity at point 'X', allowing trains to cross at that end of the loop. The relay turns on and off as the track switch points are changed from left to right. The relay and the switch machine are operated automatically by the MRD2 dual train detector, so there is no need to control the loop track polarity. Wire the switch machine so the switch points are aligned for the left track when sensor 1 is triggered. If the points align the wrong way, swap the connections to the switch machine. How it works: Assume a train is moving right-to-left on the main track, approaching the loop, and the turnout is set to the left. It enters the loop and travels clockwise. When it triggers sensor 2, the MRD2 will throw the switch points to the right and turn on the relay. The main line and loop track polarities will now match at point 'X'. The train returns to the main line. Now the next train approaches the loop, moving right-to-left. The switch points are still aligned to the right and the track polarities match at point 'X'. The train travels counter-clockwise around the loop and triggers sensor 1. The MRD2 throws the switch points to align with the left track and turns off the relay. Mainline and loop track polarities now match at point 'M' and the train returns to the main line.

4 Manual control of the turnout is possible, as shown in the MRD2 installation instructions. For DCC powered layouts and AC layouts with a single train transformer this is all you need. For DC layouts there is a catch. With DC power, track polarity determines train direction. If the turnout is manually thrown, a train moving on the main line will change direction! To avoid this, you can either: Not use manual turnout control, or stop the train before changing the turnout. MRD2 Dual train detector Includes two infrared sensor pairs (IrLEDs & photo-transistors) MRAPR Relay, DPDT, 5 amp contacts Easy installation, screw terminals and mounting holes 8 to 14 volt coil, AC or DC On-board LED indicator and kickback protection Tortoise slow motion switch machine Circuitron part no Includes DPDT contacts for frog power or signal routing Accessory power supply, PS1206 Regulated 12vdc, 600mA, slim plug-in Two supplies will power up to 10 MRD2's with switch machines $36.50 $9.15 $17.25 $9.50

5 Automated reverse loop with 'snap' switch machine If your track switch is operated by a twin-coil 'snap' switch machine, the system at right shows how to automate your reversing loop. We recommend two separate accessory power supplies, one for the switch machines and another for electronic accessories. Solenoid switch machines can draw considerable power, and cause significant voltage disturbances on their power lines when activated. If electronic circuits are powered by the same power supply, these voltage fluctuations can cause the electronics to reset or behave unpredictably. Set up the MRD2 dual train detector according to its instructions. Then test the switch -- when a train is detected by sensor 1, the switch points should align to the track with sensor 1. If the points instead align with the opposite track, swap the two outside wires at the switch machine. The latching relay switches at the same time as the switch machine. When coil 'A' is momentarily energized, the contacts move to their 'A' positions (terminal 1C connects to 1A, and 2C connects to 2A). When coil 'B' is momentarily energized, the contacts move to their 'B' positions (terminal 1C connects to 1B, and 2C connects to 2B). The contacts of the latching relay maintain their position after power is disconnected from the coil, just as the switch points maintain their position after the switch machine power is disconnected. For single-coil (2-wire) switch machines such as Kato, AristoCraft or LGB, use the diagram shown here. Operation is the same as the twin-coil circuit shown above. We show an AC power supply with two diodes for actuating the switch machine and latching relay. If your switch machines are powered by two DC power supplies, use them with this circuit instead of the AC supply and diodes. Manual control of the turnout is possible, as shown in the MRD2 installation instructions. For DCC powered layouts and AC layouts with a single train transformer this is all you need. For DC layouts there is a catch. With DC power, track polarity determines train direction. If the turnout is manually thrown, a train moving on the main line will change direction! To avoid this, you can either: Not use manual turnout control, or stop the train before changing the turnout.

6 MRD2 Dual train detector Includes two infrared sensor pairs (IrLEDs & photo-transistors) LARY-AC Latching Relay, DPDT, 8 amp contacts Use with twin coil (3-wire) switch machines Easy installation, screw terminals and mounting holes 12 to 17 volt coil, AC or DC LARY-DC Latching Relay, DPDT, 8 amp contacts Use with single coil (2-wire) switch machines Easy installation, screw terminals and mounting holes 12 to 17 volt DC coil Accessory power supply, PS1206 Regulated 12vdc, 600mA, slim plug-in Excellent power supply for MRD2 and other electronic circuits $36.50 $13.75 $13.25 $9.50