The line schemes are made by me on the base of graphics taken from Mark Hodson’s Wiki for Rail3D users.
All other pictures are taken from unknown Internet source, and show signals used in Poland. Some of them are modified by me.

Grzegorz Jagodziński

One-Track Line in Train Simulators

Part 2

The Previous part

We are approaching to a station

As it has already been shown, a typical through station on a one-track line has two tracks:

When our train is approaching to the station, the decision must be taken what track the train will enter: the main one, or the loop one. There is a rule saying that when two trains are going to cross on a station, the first train is run to the loop track, independently on whether it is an up train or a down train. The second train will go to the main track.

But when a given train is not going to cross with another train (when it is sure that it will leave the station without crossing), it will occupy the main track (as there is no need to run it to the loop track).

The entry junction must have been set for our train some time before it will appear nearby. Otherwise, it would be obliged to stop before entering the station when having the entry semaphore at sight. It would not be too comfortable for passengers rather: it is better for them to have a cola at the station restaurance rather than to hunt for rabbits somewhere in the wild, when the train is stopped by the entry semaphore. So, the rule of thumb on one-track lines is that at least passenger trains should wait for free way in stations, not in trails.

In order not to stop the train by the entry semaphore, the station-master must decide to which track he will run the train before it reaches the entry semaphore so that the train driver would see the proper signal. Note that if the main track is chosen, the signal will show free way. But if the loop track is chosen, the signal must show the speed limit.

Setting the proper signal when the train is near to the entry semaphore is not enough, though. Notice that the semaphore is preceeded by a warning shield which gives a distant signal. It will be green when the train enters the main track – but flashing orange when it enters the loop track.

Thus, in order not to make surprise to the train driver, the entry junction must be set enough early, in the time when the train is still long enough from the warning shield. The driver must have time to notice the signal on the shield and prepare to the speed reduction if needed. So, it would be a good idea that the warning shield would have a kind of detection radius in simulators, let’s say 500 m or even 1 km. In the moment when the train is crossing the radius, that is when it is entering the detection area, the decision must be taken what to do next. The entry junction must be set for the train, the proper signal must be lit on the entry semaphore, and the proper signal must be set on the warning shield. No further changes of the state of the art are possible until the train stops at the station platform.

Now we will analyse various positions of two trains nearby a given station. Let’s start from the simplest one.


In this situation, while our train is approaching to the station B, another train, the white one, has already come to the station and is waiting for crossing on the loop track.

17. Notice the moment when the green train is entering the detection area of the shield ShAB. Before the train passes the shield, the entry junction has set to the main track, the semaphore AB has shown the proper (orange) signal, and the shield ShAB has shown free way.

18. When the train is passing the shield, the shown signal does not change. It is so because shields are not semaphores, and they do not react to passing trains but only to the next semaphore. And inversely, the signal shown on the semaphore AB changes into Stop when only the loco has passed it. In the same moment also the signal on the shield ShAB changes. A good simulator should behave in the same way automatically, without the need of any user activity.

19. When the last waggon has passed the entry junction, the way for the white train gets released (instead, we could expect that the whole train has passed the BA2 semaphore, the one for the opposite direction, which protects the junction). The junction switches, and the BA1 semaphore shows free way (with speed limit, as always for a loop track).

20. The white train leaves the station slowly until it has passed the junction entirely. Then it runs faster.


This time the white train is not in the station yet but it is getting close to the detection area of ShCB.

21. It enters the detection area of ShCB, and only next the green train enters the detection area of ShAB (watch it with much attention as the animation is enough fast). In other words, the first train detected by the station B is the white one. This is why it must go to the loop track.

22. However, going to the loop track is possible only with reduced speed (notice the double orange signal on the semaphore CB which means “speed limit, then stop”, notice also the flashing orange signal on the shield ShCB which warns the driver to have reduced the speed by the semaphore). The result of the speed limit is that the green train arrives first than the white one. All is right, though. The loop track is for the first train detected, not for the first train arrived. There is no better solution. If the green train was run to the loop track, it would arrive later than the white one, and the difference would be even more. It is so because the first train has to slow down while the second one has not to. If both trains arrive almost in the same moment, we will always observe the picture like in the animation. If the difference between times of detections was more, the white train would arrive first and the situation would be like previously (7).


And this time the green train is the first in the detected area of the station B. The situation is inverse to the previous one.

23. As our train (the green one) enters the detection area (of ShAB) first than the white one would enter the detection area of ShCB, it must go to the loop track – hence the speed limit on the entry semaphore. The station-master in B must know, however, that another train (the white one) is approaching even if has not been detected yet. Otherwise it would run the train to the main track.


This situation is not very different from the previous one. Once again, our train (the green one) will go to the loop track.

24. The white train is just starting from the station C. this means that both trains, the green one and the white one, will cross in the station B. As the green train will enter the station first, it must wait for the crossing on the loop track. The station-master at B must be informed that the white train will cross with the green train on his station.

It means that when the green train is entering the detection area of ShAB, the simulator must detect not only if there is any train in B and not only if there is any train on the B-C trail (like in 9) – but also if there is a train in the next station (that is in C).

25. Moreover, it is important in what direction the other train goes. But if it is just standing in C (and waiting for its departure in the timetable) there is no need for running the green train to the loop track in B. The more there is not such a need when the white train was an up train. In either instance the trains will not cross in B. The green train (in the situation like in the picture) should occupy the loop track only if the white train is about to leave C, and if it is a down train. Only in this instance the trains will cross in B.

In other words, the simulator must be able to detect the white train in the moment when the green one enters the detection area of ShAB, and it must also be able to detect that that train goes to B (even if it is standing now – but it is going to leave C).


And once again, the situation is not very different from the previous one.

26. Both trains will cross in B, not in C. It is so because the white train has already entered the detection area of C while the green train has not entered the detection area of B yet. It means that the white one will be the first to come to the nearest station. And also the white one will be ready to go first. As the white one will occupy the trail (C-B), the green one must wait in B until the trail releases. This is why both trains will cross in B, not in C.


And finaly, the situation 12 is exactly the same as 11 except this time the green train is the first to enter the detection area. This is why the trains cross in C, not in B.

Polish Railway Signals