In the real world, reverse loops are used at the end of long section of track so that a complete train can change direction. They are used for large “block trains” which carry bulk loads like iron ore and coal. The train is unloaded at one end (usually without stopping) and then proceeds around the loop and goes back to be loaded again, perhaps hundreds of kilometres away at the mine.
The photo above is a satellite view of a real-world loop at one of the coal loaders in Gladstone, on the central Queensland coast. In fact, there are reverse loops for several coal loaders in Queensland and they used at other bulk loaders around Australia, so they are not simply a feature enjoyed by the model railway fraternity.
In the modelling world, a reverse loop (or two) on a layout will allow a train to change direction without it having to be physically picked up and swapped around. But as noted above, the model reverse loop has a serious problem which does not affect real-world railways – shorts in the track.
Note that while shorts in reverse loops are problem with all model railways, we should state at the outset that this project is only suitable for DCC layouts. For more information on DCC operation have a look at the article in the February 2012 issue and the high power DCC booster project featured in the July 2012 issue.
To repeat, this project will not work on model railway layouts which employ conventional (ie, variable DC or PWM) controllers.
Fig.1a shows how a reverse loop works in a conventional layout. As you can see, a reverse loop has one set of points (in US parlance, switch or turnout) which is set one way to allow the train to enter the loop.
It is then set the other way to allow the train to travel out of the loop, in the opposite direction along the single track with the loco still leading the train.
However, if you follow the top (red)rail from point “A” all the way around the loop to point “B” you will see that there is a short circuit. The rail (red & black) colours in the diagram highlight this major problem on any model railway layout.
Fig.1a: this simple
diagram of a
reversing loop shows
why we have a problem
- with both standard and
DCC layouts. The red and black lines represent the two rails – as you can see, regardless of which way the points are set (in this case the train is traversing the loop clockwise) there will always be a short circuit between two of the tracks (shown here with the green circle). In real life, this doesn’t matter – but for model railroaders, where the tracks supply the loco power, it is a serious problem!
Fig.1b: and here’s the solution - a relay switches the polarity of the tracks at precisely the right moment so that the short is eliminated. This arrangement would not work for standard (DC) tracks (the train would go backwards) but is perfect for DCC layouts. A microcontroller takes care of the timing.