A reverse loop sometimes called a balloon track because of its shape, allows a train to change direction without going in reverse. On the prototype, these tracks consume a lot of land as trains cannot handle sharp turns. They are used at coal mines, grain elevators, power plants and similar large industries that can load/unload an entire unit train. What they cost in space, they save in time and efficiency. Locomotives do not need to be uncoupled, turned and run around the train. Trolleys frequently use reverse loops at the ends of runs, taking advantage of their tighter turn radius.
On a model railroad, as with the prototype, reverse loops consume a lot of space but are the fastest way to turn an entire train. One of the most common uses of reverse loops in modern layout design is as part of a staging yard. By incorporating a reverse loop, arriving trains can be quickly turned and readied for their next run.
What's the Problem?
Most model railroads use the tracks to transmit power to the trains. The majority of two-rail track systems use a positive (+) voltage on one rail and a negative (-) on the other. In a reverse loop arrangement, the left rail will eventually touch the right rail and create a short circuit.
The same rules hold true for other reversing track sections including wyes and turntables. Other typical track arrangements, like a "Figure-8", look like reversing loops but aren't. If you have found a short circuit that is fixed by removing a section of track, there is a good chance you've got a reverse loop.
Three-rail track systems, most commonly associated with O Gauge trains, solve this problem by putting a common voltage on both outside rails, using the center rail for the opposite. In this way, the positive and negative rails never meet.
Fortunately, there are easy solutions to these electrical problems that don't involve adding a third rail.
Wiring a Reverse Loop for DC Operation
If your model railroad uses a conventional DC (Direct Current) power supply, adding a reverse loop is only a toggle switch away. This method will work with the control systems offered in most two-rail track train sets on the market today. The same method can be used with Digital Command Control (DCC) although there are other options for these systems as well.
These same steps work for loops and all other reversing track sections including wyes and turntables.
Isolating the Reversing Section
Before you can wire the reverse loop, it must be electrically isolated from the rest of the railroad. This can be done by inserting insulated rail joiners on each rail at each end of the reversing section. For turntables, the gaps between the bridge and ground do this automatically. For reverse loops or wyes, the gaps are most typically placed at the trailing ends of the turnout that creates the loop/tail track.
Install plastic insulated joiners, or cut gaps in both rails.
Selecting the Toggle Switch
A Double-Pole, Double-Throw (DPDT) Toggle Switch is needed to reverse the electrical current in the reversing section. These can be found at most hardware and electronics stores. Most varieties of readily available switches will be able to handle the voltage and amperage requirements of model trains. Look for something rated for at least 14 Volts and 5 Amps.
Some toggles also have a "Center Off" position. This is a good option if you would like to be able to turn off power to the reversing section entirely. Look for a switch marked "DPDT Center Off."
You can find switches with solder or screw terminals on the back. Either will work if you aren't confident in your soldering skills.
Wiring the Toggle Switch
- The toggle switch will have six terminals on the back. To prepare the switch for the reversing section, use two wires to make an "X" between the top and bottom terminal pairs.
- Attach the outgoing power to the center two terminals. These wires will connect to the track in the reverse section. Multiple feeders can be used on large reverse loops.
- Attach incoming power to either one of the other pairs. This can be a connection directly from the power supply or from the power bus that feeds the rest of the layout.
Operating a Reverse Loop With DC Wiring
Before the train can enter the loop, the toggle switch must be properly lined so that the polarity of the reverse section matches the polarity of the rest of the railroad at the entrance point. It is helpful to orient and labels the switch accordingly. "IN/OUT," "A/B," "EAST/WEST," or other notations should help operators plan the move and avoid shorts. The loop can be used in one or both directions. If the loop is going to be used in only one direction, consider adding a spring switch to the entrance to simplify operations.
For turntables, in/out may not be an appropriate designation. Some turnouts feature an operator's cab at one end. You could also paint handrails or a tie at one end of the bridge to identify an A vs. B end. Label the toggle switch accordingly. Wire all tracks connecting to the turntable parallel with the rest of the railroad. Only the table itself needs to reverse polarity, selecting A or B for the end of the bridge your engine will cross.
The toggle switch will reverse the polarity of the rails in the reversing section once the train is in the loop. When this happens the train will also reverse direction. To keep the train moving forward, you must also change the direction on the throttle. If you throw both switches simultaneously, you can do both while the train is in motion. Of course, you can also stop the train prior to throwing the switches.
It is always a good idea to leave the track turnout and the polarity switch oriented in the same direction. If you only use the reverse section in one direction, plan ahead and reset both switches when your train has completed its run through the track.
Wiring a Reverse Loop for DCC
Like conventional DC wiring, the reversing loop or track section must be electrically isolated from the rest of the railroad even with Digital Command Control (DCC). This can be done with plastic rail joiners or simply by cutting gaps in both rails at each end of the reverse track.
Reverse sections can still have their polarity flipped manually with a toggle switch with DCC. The only difference is you will not need to change the direction switch on the cab as the polarity in the rails no longer determines the direction of travel.
A second option is to install an automatic reversing unit. These circuit boards detect the short circuit when a train enters or leaves a reversing section lined against it. As soon as the short is detected, the board flips the polarity in the reverse section. The train should continue running without hesitation. An added advantage of these boards is that many also act as a circuit breaker and can avoid damaging a decoder or other expensive components.
Wiring the Auto-Reverse
While there are several versions available, most automatic reverse boards are simple to install. Most require only attaching two input wires from the non-reversing bus and two output wires to the reversing section. Some provide options to add resistors to increase sensitivity or add LED indicator lights for control panels.