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In Australia Railway signalling is known a safeworking, perhaps due to the fact that not all safeworking systems involve signals.
Most forms of train control involve messages being passed from those in charge of the rail network or portions of it to the train crew; these are known as 'signals' and from this the topic of train control is known as 'signalling'.
The simplest form of operation, in terms of equipment at least, is operation according to a timetable. Everything is laid down in advance and every train crew knows the timetable. Trains can only operate in pre-arranged time periods, during which they have 'possession' of the track and no other train can operate.
When trains are operating in opposing directions on a single-line railroad, meets are scheduled, where each train must wait for the other at a point they can pass. Neither is permitted to move until the other has arrived.
The timetable system has several disadvantages. The first is that there is no positive confirmation that the track ahead is clear; only that it should be clear. This system does not allow for breakdowns and other such problems. The timetable is set up in such a way that there should be sufficient time between trains for the crew of a broken-down or delayed train to walk back up the line far enough to set up warning flags, flares and the explosive devices known as detonators or torpedoes (UK and US practice, respectively) which alert a train crew to a blocked track ahead.
The second problem is the timetable system's inflexibility; trains cannot be added or delayed; trains cannot be rescheduled.
The third is a corollary of the second; the timetable system is inefficient. To give a little flexibility, the timetable must give trains a broad swath of time to allow for some delay. Thus, the line is possessed by the train for much longer than is really necessary.
Nonetheless, this system permits operation on a vast scale, with no requirements for any kind of communication that travels faster than a train. Timetable operation was the normal mode of operation on American railroads in the early days.
With the advent of the telegraph, a more sophisticated system became possible. The telegraph allows the dissemination of alterations to the timetable, known as train orders. These override the timetable, allowing the cancellation, rescheduling and addition of trains, and most anything else. Sufficient time must be given, however, so that all train crews can receive the changed orders.
Train crews generally receive the orders at the next station at which they stop; or sometimes orders are handed up to a locomotive 'on the run' via a long staff.
Timetable and train order operation was commonly used on American railroads until the 1960s, including some quite large operations such as the Wabash Railroad and the Nickel Plate Road.
Timetable and train order was not used widely outside North America.
Timetable and train order operation still has some significant flaws, such as an over-reliance on the ability of the crew of a stranded train to let other trains know of the problem, and a general intolerance for human error. When everything goes perfectly it works well, but mistakes are easy and deadly.
Timetable and train order is only suitable for railway lines which carry relatively little traffic, and is unworkable on busy rail lines because it requires great separation between trains. Where this is the case, physical signals need to be used (either mechanical semaphore signals, or - more commonly in the modern era - electric light signals) to show the train crew whether the line ahead is occupied and to ensure that sufficient space is kept between trains to allow them to stop.