Signal boxes were a defining change in railway technology.
The 1889 Regulation of Railways Act allowed the Board of
Trade to enforce the absolute block system, plus the interlocking
of points and signals. This resulted in operation of the system
being centralised in signal boxes, using bell communication
between boxes to allow only one train in each section, and a
mechanical logic to control points and signals. However, the
system was both heavy on maintenance, and limited in scope
by how far a single man (there were no women working in signal
boxes) could operate a pair of points or a signal, so busy
locations needed a large number of signal boxes.
