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 chek your work, you will have a fully automated reverse loop and worry-free train operation.
 
[Click here to review reverse loop fundamentals]
 
Here are two plans for a single loop:

The first system is completely automated. Trains alternate their direction of travel each time they go around the loop. A slow-motion switch machine (required) is powered by twin DC power supplies.
 
This system, shown at right, uses a DPDT relay to control polarity of the main line.
 
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.

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 controller (cab) this is all you need.
 
For DC layouts and multiple-cab AC layouts see the note below.

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,
• Stop the train before changing the turnout, or
• Use the system shown below...
 
[Click here for multiple cab control wiring info]

This complete automated control system for a single reverse loop can be used with any switch machine and is fully compatible with DC, DCC and AC layouts.
 
Track polarity is controlled for both the main line and the loop track. The turnout can be thrown manually without affecting a train on the main line.
 
Use this system if:
• You want to use a solenoid-type switch machine, or,
• You have a DC layout and you want to be able to throw the track switch manually without affecting a train on the main line.
 
Otherwise the single relay system above will work fine for less money.
 
As in the system above, an MRD2 dual train detector controls the track switch. An MRD1 single detector is placed on the main line just ahead of the switch points.
 
All detectors are connected to the MREV Polarity Controller. Track power to both the main line and the loop track is routed through the MREV circuit.
 
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. It will trigger sensor 3, alerting the MREV to change the loop track polarity according to the track switch position. The MREV gets the track switch position info from the MRD2 dual detector.
Note that sensor 3 and the MRD1 detector are only required if the turnout will have the option to be switched manually.
 
The train goes around the loop and approaches the switch, ready to return to the main line. It will trigger either sensor 1 or sensor 2. This causes the MRD2 to throw the switch, and the MREV will change the main line polarity so the train can exit the loop without interruption.
 
The MRD2 and MREV circuits have non-volatile memory. They remember the switch position and polarity settings when layout power is turned off.
 
Components for the single reverse loop control system may be ordered separately or as a package. Detector circuits and the MREV polarity controller require a power supply, 8 to 14 volts AC or DC.

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