Reverse Sections

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Reverse Sections

When you heard about Digital Command Control and all its advantages they told you that you wouldn't need to worry about polarity issues and reversing sections.
Not quite true!

Except for 3-rail track, all electric train layouts have to be specially wired when it comes to reversing sections of track. A reversing section allows an engine (or train) to enter in one direction, and leave on the same track heading back the way it came. Reversing sections include: reversing wyes or triangles, turntables, and reversing loops.

Digital Command Control has phase issues, and a reversing section will cause a dead short if not wired correctly. With DCC, Rail B is held to ground while Rail A is Positive, which then flips (Rail A is held to ground and Rail B becomes Positive) to construct the DCC digital waveform.

As DCC uses a digital signal on the rails to provide both commands and power to the mobile decoders, there is no polarity issue, as the signal consists of only logical high and low values. There is no concept of polarity with digital, as there are no positive or negative values. The phase issue is that Rail A is always the logical opposite of Rail B.

In a traditional layout, a reversing section is insulated from the rest of the layout. It is powered through an independent reversing switch. The train enters the reversing section and, whilst the train was totally inside the reversing section, the power to the main line is reversed. When the train leaves the reversing section there is no display of sparks!

This must be done with DCC too. The difference is that in an automatic reverser can automate this by throwing a 'soft' switch. As a train enters or leaves the reversing section, the reverser detects the short circuit caused by a train entering the isolated section and reverses the phase of the reversible section of track. This happens quickly enough that the train continues on its way unimpeded.

This behavior has some side effects that bear consideration. First, a single reverser may be used to control more than one reversing section. The constraint is that only one train my be entering or leaving reversing sections controlled by a single reverser at a time. Second, the reversing section must be long enough to contain all of the train that may trigger the short -- engines, cars with electrical circuitry, cars with conductive wheels. Care must also be taken that the current capacity of the booster and wiring is sufficient that the reverser detects the short in a timely manner and performs its function. If the booster or power management device acts too quickly things won't work well.

The LDT KSM-SG though is different than most reverse-loop modules, while the reversal phase of the reverse-loop will be performed without short circuit due to 2 sensor-tracks which are located at the entrance and at the exit of the reverse-loop.

Application Example 1: Reverse Loop

The AR1 is Digitrax's product to provide automatic reversing of track phase whilst a train passes through a reversing section. Although the AR1 is used in these examples and diagrams, there are many other products from various manufacturers which work just as well (and some argue better) than the Digitrax AR1. We highly recommend you check out various reversing units from various manufacturers and vendors before making your final decision.

AR1 connected to reversing loop
Digitrax AR1 Auto Reverse Module

Connection is simple via the two pairs of wires: one feeds track power to the AR1 (shown on the right, above) and the other feeds from the AR1 to the loop. The gaps in the rails are essential: they isolate the two opposite phases of electric power (see DCC Tutorial (Power)). Electrical gaps must exist in both the places shown. These gaps can be made by using insulated rail joiners or cutting the rails and filling the gaps with an insulated material such as styrene. Other devices available on the market work in much the same way.

When a locomotive bridges the gap, rather than stalling or shorting out the track bus, the autoreverser switches the phase.

Because of the double-gaps, it does not matter whether the point has an insulated or a live frog (such as a PECO Insulfrog or Electrofrog type turnout).

Application Example 2: Reversing Triangle (Wye)

Whereas reversing loops are rare on full-sized railways, triangular junctions, or wyes are quite common. The AR1 can also be used for these. The triangle may connect to a separate line (as in full-sized practice) or may simply be a convenient spur siding.

A wye was often used to turn a locomotive where a turntable was not available.

AR1 connected to reversing triangle

The track feed connects to the AR1 input just as in the reversing loop case. The AR1 output feeds the spur. The two double gaps on the right are essential because they isolate the different power polarities.

It is important to note that it is not possible to make any other feed connections to the spur other than via the AR1 unit.

The double gap on the main line marked with an asterisk (*) is required if the turnout or points have a live frog.

Reversing units

(Please use this area to list and provide links to devices which will handle reversing sections.)