An overlap in railway signalling is the length of track beyond a
stop signal that is proved to be clear of vehicles in the controls of the previous signal, as a safety margin.
Overview
Enough
braking distance
Braking distance refers to the distance a vehicle will travel from the point when its brakes are fully applied to when it comes to a complete stop. It is primarily affected by the original speed of the vehicle and the coefficient of friction be ...
is provided to allow a train to comfortably stop at the stop signal, but should it fail to do so for any reason (fog, smoke,
slippery rail
Slippery rail, or low railhead adhesion, is a condition of railways (railroads) where contamination of the railhead reduces the traction between the wheel and the rail. This can lead to wheelslip when the train is taking power, and wheelslide ...
s,
brake
A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction.
Background ...
failure, inattention by driver, etc.) there is still some distance to allow the train to stop short of any obstruction, such as another train.
Length
The length of an overlap may be a nominal length, or else calculated on the full emergency braking distance for the gradients and speeds for that particular section of line.
Two signal protection
If the overlap is long enough to extend from one stop signal to the next, then the effect is to provide two signal protection (or "double blocking"), with the first signal having a full overlap, and the second signal having a short overlap.
With two signal protection, only one
track circuit
A track circuit is an electrical device used to prove the absence of a train on rail tracks to signallers and control relevant signals. An alternative to track circuits are axle counters.
Principles and operation
The basic principle behind t ...
is needed between successive signals. However, overlaps may then be longer than is necessary with an adverse effect on headway.
With one signal protection, there may be two track circuits between successive signals, which has a cost, however headways can be optimised.
Country practice
Overlap design vary widely with signalling practice in different countries and types of
train protection system
A train protection system is a railway technical installation to ensure safe operation in the event of human error.
Development
Train stops
The earliest systems were train stops, as still used by the New York City Subway, the Toronto subway, t ...
s. Many countries do not use overlaps at all.
Germany
In Germany the following distances before entry to a station or another critical point are used:
[Haldor Jochim, Frank Lademann: ''Planung von Bahnanlagen. Grundlagen - Planung - Berechnung'', Hanser Verlag, 2008, ]
United Kingdom
(Typical values)
* Mechanical Signalling: 440 yards (402m)
* 3-aspect colour lights: 330 yards (300m)
* 4-aspect colour lights: 220 yards (200m)
References
Railway signalling
{{rail-transport-stub