A Brief History of the Signalman & Signalling.
The earliest railway signalmen were in fact police officers employed to keep order on the railway, to set the road and to make sure the line was clear for the arrival of trains. The policeman would have had to walk between points to make sure that they were set correctly and used hand signals to communicate with the drivers.
The Great Western Railway in 1838 issued instructions to their drivers and policemen about signals. All clear was signalled by holding the right arm horizontally, the arm was held up for caution and Stop was signalled by holding both arms above the head.
At night coloured lamps were used, All Clear being a white lamp, green being Caution and a red lamp for Stop or Danger. As well as hand signals during the day coloured flags could be used, colour coding being the same as the lamps used at night.

Hand signals were gradually replaced by fixed signals i.e. it did not move as a policeman would have. These signals were easier sighted by the engine drivers as they knew where they were located and could be seen from further away. Fixed signals were at first worked from a lever at the base of the post which still meant a lot of walking for the policeman. At larger or more complex stations the policeman was assisted by pointsmen who would work points and signals under his instruction.

Trains were at first signalled between stations using a time interval system. This basically worked by letting one train go and the waiting a set time before allowing the next one to proceed. This system was quite safe in the early days when traffic levels were low, the only problems arising when a train failed between stations blocking the track, it was at this point the train guard was meant to walk back far enough to warn the next train, allowing the driver enough time to stop (brakes were very limited at first). If a train was delayed in the station for any reason then the driver of an approaching train would have no idea to the circumstances. This led to the use of distant signals to give the driver advanced warning of whether the station was clear or if there was still a train blocking the line. These signals were worked by wires connecting to a lever at the station, this was the first step towards concentrating the operation of signals to one place.

Fixed signals were an improvement on hand signalling but being worked from the base of each post and the same with the points could be very labour intensive work. With the railways becoming busier it was a natural progression to use wires and rods to work the signals and it then became practice to group together the levers to work all points and signals. At first these levers would have just been housed on a platform open to the elements but in time they were covered with a cabin and the basics of the signal box came together.

Concentrating all the levers to operate a station in one place enabled the railway to dispense with pointsmen, saving many hours labour and allowed the development of interlocking between signals and points. Interlocking of signals and points to prevent conflicting movements started from the 1840’s but took a long time to be installed in every signal box. The main reason for this was the cost, such interlocking could be very expensive and to a small railway company, with limited funding, who only operated one train on the line at any one time, considered as unnecessary expense, especially as it didn’t bring in any extra revenue.
An accident in Armagh in 1888 led to the Regulation of Railways Act 1889 which enforced the requirement for all points and signals to be interlocked and also the removal of the time interval system of working.

Leading on from the Time Interval system, the invention of the electric telegraph made it possible for stations to communicate with each other. This meant that the policeman could ask the next station if the last train had arrived before sending the next one adding another layer of safety. This system allowed the trains to be separated by space instead of time, which is the basic theory behind “Absolute Block Section” working.
Developments in technology brought into use specialist equipment for the sole purpose of  keeping trains separated by space, or Absolute Block Section working as it became known.
At this point it is worth defining the definition of an “Absolute Block Section”. In railway signalling terminology an “Block Section” is described as being a section of single line between the outermost controlled signal of one signal box to the first controlled signal of the next signal box. The principle of Absolute Block Working is that only one train, at any one time, travelling in one direction, maybe permitted on a single line. I.e. only one train can be on the line at a time.