The distant signal is arguably the most important development in railroad signaling history. It marks the emergence of the concept of safe braking distance, and with it a focus on safe operation at speed.
The railroad is a low-traction environment. This is responsible both for its energy efficiency and its long stopping distance. The safe braking model dictates that a train must be given at least enough distance and time to come to a stop between a stop or danger indication and the obstacle responsible for the indication.
The braking curve is the most important assumption made in the design of a train control system. It dictates how fast a train can be expected to decelerate, and thus dictates the control lines of every signal, or how far ahead the signal must look for obstacles.
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diagram of control line and signal scenario diagram, showing braking curve
The length of the signal control lines dictates capacity, as longer control lines (associated with higher max speeds), mean more of space on the line is dedicated to maintaining train spacing at any given time.
to prevent trains from crashing into obstacles, enforce a distance of at least one safe braking distance from the maximum possible speed at that point
to control braking distance, control speed
automatic signal, blend of distant and main