South Downs Railway

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Power Generation & Control

Reverse Loops

Many railways include what are called "reverse loops". These are track constructs that allow a train to urn around and come back the way it arrived. These are fine operationally, but need some care when providing the power to them.

The problem with a reverse loop is obvious if you draw out the two rails: they end up shorting out. With a DCC powered railway, the standard solution was to stop the train in the reverse loop, then reverse the power and drive it out with reversed polarity. DCC provides an alternative: the "autoreverser".

An autoreverser is simple: it feeds power to a track section, with an ability to reverse polarity to that section. The reverse loop is powered in three parts: each end is fed according to the track leading to it, while the centre section is fed by an autoreverser. When the train either drives into or out of the centre section, a short occurs when the wheels first cross the gap. The autoreverser detects the short and quickly swaps over the polarity (typically with a relay). That releases the short, allowing the train to continue.

For this to work, the centre section needs to be larger than the longest train that has its wheels connected through.

Digitrax provide an AR1 autoreverser, which is comparable to several other products. Its PM42 quad power manager can also have its outputs individually programmed to be autoreversing. Some claim the PM42 has problems when being used for autoreversing but I have seen no evidence of that with several reverse loops on my railway.

One thing to remember is that an autoreverser is not a power manager. If a short occurs in the reverse loop, the autoreverse will keep changing polarity to try to clear it but will not succeed. It is possible to use a power manager then an autoreverser to provide both functions; that would use two of the 4 zones on a PM42.

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