Two hundred or so years ago every town and city in the UK had a different time. For example, if it was 11.00am in London, in Bristol, which is 200 miles to the west, it would be 10.50am. This is because each had their own time according to a local sundial. Local time had worked for hundreds of years – right across the world in fact!
When the railways started running, a railway timetable was introduced, as trains need to run on a timetable. This meant there could only be one time, from which everything would run from. That time in the UK was Greenwich Mean Time (GMT). The time signal for this ran from the Royal Observatory in Greenwich, London, along cables that ran alongside the railway lines to every station in the UK.