What are signal boxes?
Signal boxes are shelters or small buildings constructed specifically to:
house the levers and other control equipment used to safely space, route and locally coordinate railway trains; and to
provide signalling staff with a vantage point from which to safely observe and efficiently control nearby trains.
Why are signal boxes necessary?
The short answer (kindly supplied by Dave Harris) is:
"to maintain a safe separation of trains and to facilitate shunting".
The more detailed answer involves a whistlestop journey through railway history and the key principles of railway operations.
1: Train drivers can't steer their trains
Trains run on (and are steered by) a pair of parallel rails laid on the ground. This means that they cannot steer round one another like road vehicles can. In places where trains need to pass one another (or to change from one route to another), points are installed and these need to be operated carefully to avoid derailments and collisions.
2: Keeping trains safe is harder when things get fast, busy or both
On slow-speed routes with just one train in operation, train crews can stop at points and use an adjacent lever to set the points for the route they want to take. This practice works reasonably well for small-scale industrial railway systems and for shunting in small goods yards, etc. However, for long-distance routes, large industrial complexes and major goods yards, this procedure is highly inefficient. Also, coordinating the trains becomes difficult if more than one train is operating in a given area. Furthermore, as the number and speed of trains increases, so does the risk of a collision.
3: To avoid collisions, rules and procedures are needed
During the early years of railway operations (more than thirty years before the Settle-Carlisle line was constructed), a series of rules and operating procedures were introduced to help things run efficiently (and, with luck, safely). For example:
Trains theoretically operated to fixed timetables and there was a fixed minimum time interval between trains using the same line. (For the Midland Railway in the 1840s, the minimum time interval was ten minutes.)
Train drivers were required to maintain a minimum distance from the train in front at all times. (For the Midland Railway in the 1840s, the minimum distance was 800 yards.) Train drivers were also required to adhere to the prescribed timetable.
A 'pointsman' was stationed at each important set of points to ensure that they were set correctly for each train.
Policemen were assigned to key locations to enforce the rules regarding train separation and safe operation.
4: To avoid collisions, train drivers need information
Initially, the railway policemen used hand, flag or lamp signals to tell drivers when to stop and when it was time (and theoretically safe) to depart. This activity is depicted in the engraving of Derby station below, where the man wearing a top hat (bottom-right corner) is holding a flag.