This page is meant for those of you folks who are not familiar with railway operations. Also this page introduces the German operations as far as my knowledge goes; other railways such as British or North American operate on different principles.
An excellent source about the principles of German railway operations is the web site of Prof. Dr. Joern Pachl.
If you are used to street traffic but new to railways, you need to bear in mind that there are two substantial differences to street traffic: The first is that the person regulating the train's speed is different from the one setting the train's track, and that the brake distance is nearly always greater than visibility. (OK that's a thing common to railways and streets, but on railways it is intended to be that way).
The other major difference lies in the meaning of a green signal: On a road, green means something like 'enter intersection on sight and proceed if clear (there may be other cars in front of you...), while in railways operations a green guarantees that the next block is clear, i.e. it conveys not only the permission to proceed, but also the statement that it is safe to do so.
That means that the train driver has to rely on the information given to him regarding whether a track is clear or occupied, and that there must be an advance warning if a signal shows stop or reduced speed, since when the driver sees the signal it is usually too late to start braking. You cannot drive along a line and start braking when you see an obstruction: instead it must be made sure that the next section of the line is clear, i.e. that there is enough distance between to trains travelling on the same line behind each other, that there is no cars standing on the line (as a train might have lost a car), and that you never ever have two trains travelling opposite directions on the same section of the line.
While keeping the distance could be accomplished by electronically regulating the speed of all trains according to their position (which is a system that is thought of nowadays and is called movable block), this was not feasible when railways were invented. A far simpler system was introduced: You divide the line into sections (called blocks) and you ensure that at any given time only one train occupies that block. This system is a bit too simple however: Since a train may stop at the beginning of a block due to unforeseen circumstances, or it might lose a car, you must ensure that either the preceding block is clear, too, or travelled at a safe speed, otherwise a following train will likely come to a halt shortly after the stopping train's position, which conflicts with the law that not two solids can occupy the same space, thus causing some inconvenience for subsequent railway operations...
To ensure the safety of operations regarding the fact that humans may (and will) err, more ore less sophisticated rules and devices have been invented, from swinging lanterns or flags, semaphore signals to modern cab signalling.
So instead of having traffic lights that simply show stop or go, you need some additional information: At first you must be pre-warned if you are approaching a signal showing stop, because otherwise you will not be able to stop on time.
Also at points you may not know whether you take this or that way (to say: the point is set straight or curved) it may be necessary to indicate that as well, since the allowable speed may depend on the set route.
Railways have developed different ways to accomplish that: To give an advance warning of the status of the next main signal (signal in advance), i.e. that signal's aspect, there is either a distant signal, or a main signal before that signal (signal in rear) may have given e.g. a speed limit, so that the signal in advance is met at a safe speed. Also some railways use track signalling, i.e. the driver is told which way he will turn, other railways don't.
Note that a main signal is a signal that is able to show stop. There may be other signals that can show things like 'slow' or 'expect stop at next signal' as their most restrictive indication. A block must (usually) be guarded by a main signal although on some especially low-speed urban railway sections there may be blocks that are guarded by main signals that have 'stop, then proceed on sight' as their most restrictive aspect.
A train movement is a movement that is timetable scheduled and is meant to
leave the station and cross into the open line, while a shunting movement is
occurring within the station limits (or the station's shunting limit signals).
However, there are a few exceptions to this rule:
As example in former rule books a red signal meant stop (and stay) for train movements but was to be completely ignored for shunting movements. In turn, at Austrian Federal Railways (ÖBB) the shunting signals always show halt when a train movement passes (which is to ignore the shunting signals).
On German Railways for a passing train movement all line-close (=shunting) signals are
set to clear.
For a shunting movement the main signals are to be disregarded if they show anything different from non-overridden stop, i.e. the movement may proceed if either the main signal is switched off (with marker light) or it shows stop with the attached shunting signal set to 'shunting permitted', but on stop alone it must halt.
Please note this is for newbies. Only true experts (which certainly I am not) may dare to take a look at German Rail's rulebook which I think were made by The Conspiracy to drive brave people insane...
The signals just indicate that a track is clear occupied or that there is a speed
restriction (e.g. because of a points area). A green signal gives a permission to
enter that block or generally speaking that the next block is technically clear
for entering into.
However this does NOT mean that the train is intended to enter that block, there may be a shunting operation the train may have been re-scheduled or anything else may apply. A train may enter a block on order only.
For train and shunting movements there are different prerequisites that a train is allowed to proceed.
For a train movement
For a shunting movement
With shunting movements, there is not only the signalman and the driver, but also the shunter (Rangierleiter, I'll check if this is the correct term that's the guy who supervises the shunting movements) involved. A shunting movement is not done on a scheduled basis, but on a special agreement between the shunter and the signalman. The signalman determines if a section's shunting signals can be technically set to clear, i.e. there is no conflicting track set up, so the signalman permits the movement by setting the signals or by verbal order. The shunter must make sure there are no derailers (and no personnel) on the track, check the vehicles brakes etc., and then orders the driver to proceed.
The difference to train movements is, that the driver may enter a section only if permitted and ordered to so, but not on a permission only. Also main signal clear aspects are NOT valid for shunting movements, but main signal stop aspects are unless overridden by a Hp 0+Sh 1 combination, i.e. a permission to proceed for shunting movements is only given by shunting/line-close signals. There are lots of shunting and protection signals some of which are permissions or orders see there.
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