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Model railway reversing loops - how to get them right

Model railway reversing loops are a useful part of any layout, allowing trains to change direction without having to travel in reverse. They can be simple or complex, but all reversing loops have a few basic components.

There are only 3 components in a model railway reverse loop

The first is the entry track, or mainline, itself.

This can lead from a station or scene break such as a tunnel or cutting.

You may even have two reverse loops connected by a single track forming a dog bone shape .

The second component is a model railway turnout. This turnout controls the direction of travel around the loop and must be placed at the end of the track.

The loop track can be a simple "balloon" shape or a more complex winding route.

Picture showing a drawing of a reverse loop
Notice how the polarity of left hand and right hand tracks reverse in the loop

Picture showing how to join the points in a reverse loop
Power to the entry turnout (points) comes from the main line

The Reversing loop Problem

You cannot simply connect all the tracks together as this will cause a short circuit.

You must have insulated rail breaks at each end of the loop
The short circuit happens because as the rails curve round in the loop they effectively swop sides and when the re-join the turnout (or point) the positive and negative rails would touch each other.

Some kinds of point require special wiring, you can learn more about that in our article Model railway points -All you need to know in one place

The good news is that there are ways to solve the problem for both analogue DC and digital DCC railways

Analogue reversing loops can be controlled by switches
Lets examine the analogue solution first using switches and/or relays

Reversing loop analogue solutions

image showing joins with a reversing loop
Start by ensuring you have insulated rail joiners or gaps at the entrance and exit point of your loop

Analogue solution #1

The first approach assumes that you want a continuous run and relies on a point operated switch to trigger a double pole double throw change over relay to switch the polarity of the main line

Image showing a loco entering the loop
The electrical connections needed to enter the loop

When you change the direction of the points the auxiliary switch on the point motor triggers the change over relay that swaps the feed over for the main line. 

Note the mainline polarity has changed
This solution requires you to swap the connections to the main line and this might not be practical depending on the rest of your layout.

Analogue Solution #2

On the real life prototypes of our model railways most of the passenger carrying ones had signals controlling the entry and exit points of junctions.

Although the signalman would always try to clear the signals for a non stop run, sometimes  this was not possible and trains would be held at a home or stop signal.

This second solution makes use of this and allows us to operate our railways more realistically, although it requires slightly more preparation.

Signal controlled reversing loop
Note the extra isolated section at the end of the loop

This method requires a second isolated section at the exit of the loop, the next question is how long should that section be?

If you have no inertia simulation on your controller and no flywheels on your locomotive then it must be as long as your longest locomotive.

Otherwise, it needs to be long enough to bring your engine to a satisfying controlled stop.

At its most basic, a point controlled change over relay can switch this section from being powered from the loop line to being powered from the main line.

Reverse loop end section
Use a double pole toggle switch or point triggered change over relay to reverse this section at the exit of the loop

Drive your train out of the loop, but you will need to change direction on your controller first

When it started its journey of locomotive had negative under its left wheels and positive under its right wheels, now with it facing the other way that has been swapped over, so in order to continue going forwards from the drivers point of view your need to change direction on the controller.

Please remember that with this solution you cannot change the points and reverse the polarity of the exit section until the engine is brought to a stop on it.

Of course you don't actually need signals for this to work, but it would look nicer. 

The same switch that powers the change over relay could also be used to trigger the signalling.

Reversing loop DCC solution

Applying a solution that works seamlessly with DCC is more of a problem.

In principle the requirements for the isolated joints at the entry and exit of the loop remain the same.

Here is the problem.... A DCC controlled locomotive requires a continual signal from the controller to remember its commands and status.

If you do not have any sound fitted locos then the solution of bringing the locomotive to a stand and then changing the polarity of the exit section will work, although with DCC you won't have to reverse the direction of the loco.

DCC Solution for Sound Fitted Locos

There are special modules called "Auto Reverse Modules" for DCC reverse loops. These devices will only work on DCC layouts and incorporate circuitry that maintains the signals to the locomotives while automatically reversing the polarity of the loop section.

The AR01 unit from NCE has block detectors built into it that detect the current draw as the train enters the loop and travels round it so you don't need  to switch it manually or by relay.

The AR01 has an input connection from the mainline and an output connection for the loop.

(other auto reverse modules are available)

Entering a DCC Reverse loop
On entry the reverse loop polarity matches the input direction

On exit the reverse loop polarity matches the output direction