Schematic of signal block section. Each section or block as it is often called is protected by a fixed signal placed at its entrance for display to the driver of an approaching train.
Most railways chose something like 10 minutes as a time interval. The lower speed trains can run closer together so more trains can be operated over a given section of line. The multi-aspect signalling commonly used in the UK today is a 4-aspect system. We use little files - cookies - that we store on your computer to help personalise our website and allows us to keep track of how you use it.
As we have already seen, a train cannot stop dead. It is provided by positioning the signal some way before the entrance to the section it is protecting.
What are smart meters? Bex and Dan learn all about the future of Britain's railways, from signals to trains and tracks! Photo of typical British railway 4-aspect colour light signal showing a double yellow aspect. Railway signalling is a complex and fascinating subject. Another safety feature introduced in the mid-19th Century was mechanical interlocking of points and signals. Even if they could rely on their trains not to make unscheduled stops and all to travel at the same speed, the 10 minute time interval restricted the number of trains which could run per hour in this case 6 over a given line.
If the section is clear, the signal can show a green or "proceed" aspect. Even with the old time interval system, the basic rule was to divide the track into sections and ensure that only one train was allowed in one section at one time. They showed a green light at night if their related stop signal was also green or clear and yellow if the stop signal was at red.
Originally, distant signals were semaphores, like the stop signals mentioned above.
Nowadays for signalling purposes, trains are monitored automatically by means of "track circuits". The above provides a simplified description of the track circuit.Railroad 101: Signaling Systems
The interlocking was performed by a series of mechanically interacting rods connected to the signal operating levers in the signal box. This block provides the safe braking distance for Train 2. This diagram right shows how the track circuit is applied to a section or block of track. This situation could occur if the train in front was standing just ahead of the signal protecting it. This was rarely the case, so a system of "distant" signals was provided in many locations.
The current flow will be interrupted by the presence of the wheels of a train.
Figure 2: The next train will be made to wait until the train in front has cleared the section. Positioning depended on the visibility, curvature, maximum permitted line speed and a calculation of the train's ability to stop.