Interesting discussion on Czech signalling.
In the Netherlands, from an operational point of view, there are 2 kinds of signals:
- Controlled signals. The signaller can set it to show stop, proceed (but it will still show red if the track is not clear or a conflicting route is already set) and drive by sight, for call on. By default they are set to stop. Since the 1990s, they are computer controlled, so if a train is expected according to the time table and no conflicting routes by earlier (but delayed) trains are expected, the computer reserves the track and sets the signal to proceed. After the first axle of the train has passed the signal, it reverts to stop.
- Automatic signals. The signaller has no control over the signal. It just shows whether the track is clear to the next signal. By default these show clear. However, when a train stops at a platform just before the automatic signal and there is a level crossing directly beyond it, the signal may show stop until shortly before departure of the train to prevent the level crossing from closing for too long.
Both kinds come in different versions, depending on the speed aspects they have to show, or for the distant signals, dwarf signals and some special signals applicable only to some (heavy, underpowered) trains. Controlled and automatic signals look identically, apart from an ID plate.
When a controlled signal is set to proceed, it makes a directional reservation of all track until the next controlled signal. Points must always be protected by a controlled signal. When showing proceed, the signal also indicates the maximum safe speed at which the next signal can be approached. The end of the track is considered a red signal, there also exists a fixed sign to be interpreted as a distant signal at caution.
A special version is the locally controlled safety system, used on some (but not all) single track railways. When the driver or guard of the train operates the button, key or infrared remote control at one station with a passing loop, then the decentral safety system checks whether the track is available, reserves the track up to the signal at the next passing loop, sets the points, closes the level crossing (if any right after the signal) and clears the colour light signal. This is somewhat similar to a token system in the sense that it can be operated by the train staff.
All other lines use centralised traffic control, mostly with absolute or permissive colour light signals, but a few lines have been equipped with ETCS-2 (cab signalling only) or dual systems: two different systems on the same line, which never give conflicting information, so that trains can choose which system to use depending on top speed and installed equipment.
Behind a train the reservation is freed immediately. So, when a train has passed some points, then another route over those points can be reserved immediately for another train, speeding up operations. There is no need to wait until the first train has passed the next signal. This means that when a train reverses, the driver can't know whether the track has been reserved for him. It may be unreserved or even reserved for a different train. Therefore he has to drive by sight and check alignment of the points, until he can see a signal telling him to proceed. This arrangement is used in Belgium too. At every location where it's likely that trains reverse, controlled signals have been placed, which in practice means at the ends of all platforms of major (junction) stations, the ends of most passing loops and in yards, but not at minor stations.
I don't really know about signalling in the Netherlands before 1930. Practices were largely similar to British practices I think, although I'm not aware of any large-scale token block working. I think it was mostly flag or semaphore signalled single track with electrical (or optical) telegraph, or signalmen walking from one signal box to the next to make sure the track was free. There were only a few trains per day anyway. Development of the dutch rail network was slow until 1860.
I have found one painting though, made around 1845, showing a train on the then single track main line from Amsterdam to Haarlem. The signaller waves a white flag to instruct the train to proceed. An electrical telegraph had just been installed and an optical telegraph was still present. It looks a bit like a semaphore signal with tilting discs, but it was for communication with the next block post, not with the train driver.
In 1933 automatic block signalling was introduced, along with colour light signalling. This also allowed bidirectional working, so that on a double track line trains could either use the left or the right hand line. Before that, trains of one company always had to keep left and trains of the other company always had to keep right. Nowadays, bidirectional working is possible almost everywhere.
Signals and points where mostly power operated by 1950, maybe before that. From then on the mechanical interlocking mechanisms where replaced by electrical relays. Every station and yard still had to be staffed, as no cables for remote control had been installed past yard limits. In the 1970s there was a brief experiment with radio controlled signalling with spring-loaded points, which depended on verbal instructions by radio (some of the last mechanical signals were removed), but after a large rail crash in 1980
, locally controlled colour light signals were installed after all. From then on things have been centralised. After 1990 everything has been computerised. Signallers only have to act to set signals to drive by sight (aka call on) or to change the routes or chronological order of trains to something different than scheduled. Nowadays there are only 13 control centres left, but some people think that's too few. If there is a problem in one of them, not only an important junction is disabled, but all bypass routes along with it, stranding half a million passengers. People are trying to get some flexibility, so that stations or complete lines can be transferred from one control centre to another when the need arises, or just during the night.
Concerning Simutrans, this means that
- Track is reserved up to the next signal, past the next stop. If the train reverses you can't really reserve past the platform (to which
signal?), so although realistically it should be reserved to the next signal, you can't in Simutrans. That doesn't really matter as that only allows few additional movements and only if no signal is build at the end of the platform, but at stations where trains reverse there should normally be a signal at the end of the platform.
- Except when using a token block system (which is not used in the Netherlands), trains revert to drive by sight when reversing. Other countries may have different practices though.
- Tied to the statement above, except when using a token block system, track is released as soon as the train has passed.
- There would be two kinds of signals, the controlled signals (or absolute signals, or whatever you call them) and the automatic signals (or permissive signals, or whatever you call them). Track reservations run from one signal to another, direction reservations from one controlled signal to another controlled signal. A controlled signal can only be cleared when there is a reservation to the next signal (or end of line, or reversing stop) and a directional reservation to the next controlled signal. A directional reservation cannot be made if a tile already has a directional reservation from a different tile, but it may have a directional reservation to a different tile, that is, directional reservations may branch, but not merge.
But directional reservations are not extremely important. Single track sections with only a single block signal can already be made and single track operations with more intermediate blocks are rare, except for emergency use, which doesn't happen in Simutrans.
Apparently, the first versions of track circuit block signals in Sweden (from 1920 and onwards) got turned completely of when no route is booked.
A number of years ago I saw something similar at a block signal on a single track line of the Rhätische Bahn in Switzerland. When the block was free, it showed green, when occupied, it showed red, when the direction was reversed, the signal was dark.
I will have to give some thought to the question of reserving beyond a station stop: it may be that it is an important thing for the purposes of priority reservations (which already has about half of its code: these will be coloured in yellow) that allow some trains to take priority over another by making a special type of reservation ahead of the normal reservation, which can be overridden by a train with a higher priority level (or the same priority level if a block reservation is requested over a priority reservation). The plan is for both the number of blocks ahead and the priority level to be customisable by line/convoy in the schedule. However, I worry a little that this will increase computational load in circumstances where some users are already reporting a higher computational load than before I implemented the signalling changes.
Interesting idea to allow high-priority trains overtake low-priority trains. Until now we needed either low speed tracks on loops with some weirdly placed signals and then hope the fast train would be so close behind the slow one that it would overtake (which meant it was already slowed down by the slow train), or use specially crafted timetables. I tried that on a few occasions, but the downside was that I needed a simple station, which looks silly, only works on straight track and forces trains to wait at least the minimum loading time, which with 15 minutes was far longer than the typical waiting time of 5 minutes.