Reverse Loops and Triangles

This article first appeared in the November/December 1988 issue of AMRA's 'Journal'.
By Steve Chapman.

Two track formations which many people have trouble with when it comes to wiring are reverse loops and triangles. The reason that these formations give problems is that at some point around the formation the feed rail and the return rail meet thus giving the opportunity for short circuits if adequate care is not taken.

Many people who wire formations of this type use a reversing switch on some section of the track to reverse the polarity after driving the train onto the section, stopping it, and resetting the points. This type of solution still leaves the operator with the opportunity to leave a switch set in a wrong position and run a locomotive across a gap onto a section with the polarity incorrectly set.

This article gives solutions to each of these wiring problems which avoid the need for manually setting the polarity of the track.

The Return Loop.

A simple return loop is shown in figure one.

figure one track diagram
figure one circuit diagram

To wire a return loop (whether simple or complex) so that a train can be run around the loop non-stop requires that the controller be of the type that has a reversing switch (preferably without a centre off position) rather than a two directional knob. All of the layout with the exception of the return loop is wired normally, this includes the section of track approaching the loop labelled A.

Section B the main part of the return loop (which to run around the reverse loop without stopping must be several times the length of the longest locomotive(s) which will be required to haul a train around the loop) is wired via a bridge rectifier (or four diodes wired to resemble a bridge rectifier) to section A. If common return wiring is used section B is NOT connected to the common return.

A short section (slightly longer than a locomotive) is connected via a single diode to section A as shown in the diagram.

 

Operation of the return loop is now straightforward. A train proceeds onto the return loop in the forward direction. Once all of the locomotives on the train have entered section B the reversing switch on the controller is thrown. Because section B is set up for travel in one direction only via the bridge rectifier the only effect that throwing the reversing switch should have (if any) on the train is a momentary disruption to the current which hopefully will not even be noticed. Once the rear of the train has cleared the points then the points can be thrown ready to receive the train back in the other direction.

If the train reaches the short section at the departure end of the loop before the points are thrown then the train will stop because the section is isolated by the points. If the reversing switch is not thrown then the train will also stop at this position because the section is only powered via the diode when the direction control is correctly set.

 

Complex reverse loops (ones with tracks coming off part way around the loop) can be similarly wired by working out which part of the track around the loop is most usefully treated as the reversing section and wiring it as shown.

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The Triangle.

A triangle junction is shown in figure two.

figure two track diagram
figure two circuit diagram

To wire a triangle requires first of all to determine whether or not there is a straight through route and to determine which of the three tracks feeds the least significant track in so far as through running is concerned. The triangle can then be wired so that the selected track will have its forward direction reversed depending on which of its two approach roads is selected.

Two changeover switches need to be set up so that they operate in conjunction with the set of points labelled X. All of the tracks fed by this approach to the triangle (sections C and D in the diagram) have their track feeds fed through this double pole changeover switch and are completely isolated from the main part of the layout except through these switches.

 

Operation of the triangle can now proceed without needing to think about track polarity except in so far as which way through the triangle that the train is to go.

Running a train in either direction between sections A and B can be done as if the triangle was not even there just by setting the appropriate road through the triangle.

Running a train between A and C (in either direction) can be done by setting up the appropriate road and considering the forward direction to be as shown on the arrow next to that path on the triangle.

Running between B and C is similar to between A and C except that the forward direction for all of the tracks attached to section C etc has been reversed and the direction control on the controller should be set appropriately.

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The Dogbone Crossover.

A variation which looks like two reverse loops overlapped appears when you put a crossover between the tracks of a dogbone shaped layout.

A proper analysis of this complex reverse loop should lead to the conclusion that the correct piece of track to consider as the reverse loop is the section of track between the points that make up the crossover. This section of track is obviously not long enough to wire as we discussed earlier and may also need to be traversed in both directions. A different solution is obviously needed.

figure three track diagram
figure three circuit diagram

The solution is obtained by considering the type of use for which the crossover has been provided ie to shunt a train across from one track to the other (if you don't intend to do this then you probably don't need the crossover). When you are shunting a train through the crossover it is obvious that another train cannot be run through on the mainline section labelled C (it is in use by the shunting train) so the solution is to make this the section of track which has its polarity reversed when using the crossover.

Again as we did to when considering the triangle formation we can tie the reverse the polarity of a piece of track to the operation of points using changeover switches. This enables section C to have polarity to match sections A and E when not using the crossover and polarity to match D when the crossover is to be used.

An additional changeover switch is used to isolate sections F and G when the polarity of C is set to match D so that no problems will be encountered by trains running across from a section of one polarity to a section with the polarity reversed.

These short sections of track (F and G) which should be slightly longer than the longest locomotive(s) used to haul trains are appropriately located to act as isolating sections to hold trains at appropriately placed signals alongside the track, but I'll leave discussion of that until another time.

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Copyright Stephen Chapman