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Reflections on the detection of the model train

In order to simultaneously run several trains on a model railroad network, it is necessary to detect the presence of a train on a canton, or its transition from one place to another. The very best is to know what train is at what place… but it’s not easy.

model railway trains
More simply, it must be possible to detect the presence of the train in a canton or its transition from one to the other. The purpose of this article relates to the detection of a passage of a train. I want to show in this article the advantage of detection of the last car or wagon – fitted with a magnet – by a reed switch.

Case study 1

Imagine the case of a long and slow model train and a system that detects the head end. This can be a reed between the rails, and a magnet under the locomotive, or an infrared barrier triggered by the passage of the model railway trains.

The direction of the flow is from left to right. If the front of the train is a long slow train and the following one is a fast train, there is a significant risk that the second catches up first. Indeed, with this passage detection system, the block is considered free as soon as the locomotive of the first train passes the detection system.

This can be remedied as follows:

1. Move further away the detector from the sensor signal: greater part of the train has left the block during the liberation of the former, reducing the risk of catching, especially with a long block. However, this does not guarantee security if the first model train is really too long and slow. On the other hand, a short train will long past the signal that will still shows way is open (although the situation is not so dangerous, the second signal being always red).

2. Delay detection by a small electronic device that is waiting a while before releasing the previous block. The risks are relatively the same as in the case described above.

3. Detect the end of the convoy (see case study 3).

Case study 2

In this second case, the detection by the reed is replaced by an infrared barrier. This naturally detects the entire model railway trains. Imagine this time a long, slow train, followed by a short, slow train, itself followed by a rapid model railroad train.

As soon as the locomotive of the first train reached the detection point, the signal that it just passed turns red and the one behind it turns green. The second train can go start again.

When the second train reaches the detection point, it will try to turn red the signal he has just left, but this red will be simultaneously countered by the detection of the first train, still active who will want to turn green this signal. However, the previous signal will have been released.

model railroading
If the train that is following is fast, it will have enough time to go up the block and will be at the same time as the second slow train in the block. This second train will be stopped at the signal and catched up by the rapid one.

To remedy this situation it is possible for example to:

1. Insert a timer or move the sensor signals. It is conceivable, but it does not guarantee that the above case can not arise.

2. Block the detection of the previous signal, as the detection is activated. This solution will indeed prevent to get two trains in the block, but will not prevent the fast model train to catch the tail of the slow one. Moreover, you will then to manually release a block.

3. Add a small electronic device which gives a pulse after the passage of the whole model railroad train.

4. Detect the end of the train only (see below).

Case study 3

In this third case study, I would like to show the situation when this is the tail of the train that is detected. This can be done with a reed and a magnet in the last car. Of course, this requires to always know what wagons and cars are equipped with a magnet. Given the simplicity and the benefits it brings, it seems to me that this is the simplest solution.

Whatever the length and speed of the first train, it releases the previous train only when the whole train has passed the detection. No risk of catching or having two trains in the block. Moreover, it also ensures to maintain the block occupied if a part of the process was to detach.

This is in my opinion the simplest solution for automatic block if the trains are not driven by the engine and when one does not want to move to a consumption detection.

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model railway trains