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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.
 
[Click here to review reverse loop fundamentals]

loop wiring with relay 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. The switch machine is powered by the D2T-2W or D2T-3W dual IR detector.
 
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.
 
• Click here to learn how a relay works. [pdf]
 
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 D2T-2W (or -3W) dual train detector, so there is no need to manually change 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 D2T-2W (or -3W) 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 D2T-2W (or -3W) 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 D2T-2W installation instructions and the D2T-3W 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.
 
D2T-2W Dual train detector for 2-wire switch machines
  such as Kato, Aristocraft or LGB.
• Also for slow motion Tortoise, Cobalt or SwitchMaster.
• Includes two infrared sensor pairs (IR LEDs & receivers)

$39.00

Include infrared sensors?
D2T-3W Dual train detector for 3-wire twin-coil switch machines
  such as Atlas, Bachmann, Peco, Kemtron, etc.
• Includes two infrared sensor pairs (IR LEDs & receivers)

$39.00

Include infrared sensors?
MRAPR-9v Relay, DPDT, 8 amp contacts
• For 9 to 14 volt accessory power supplies, AC or DC
• Easy installation, screw terminals and mounting holes
• On-board LED indicator and kickback protection

$9.50

MRAPR-12v Relay, DPDT, 8 amp contacts
• For 12 to 17 volt accessory power supplies, AC or DC
• Easy installation, screw terminals and mounting holes
• On-board LED indicator and kickback protection

$9.50

Tortoise™ slow motion switch machine
• Circuitron part no. 800-6000
• Includes DPDT contacts for frog power or signal routing

$17.50

Accessory power supply, PS1206
• Regulated 12vdc, 600mA, slim plug-in
• Can power up to 8 D2T-2W circuits with slow motion switch machines

$9.50


[ Choose a different type of reversing track ]


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