In 1840, Great Western Railways set a standard time on its lines. Over the next seven or so years, this was taken up by other railway companies. London time was used, which was the time at the Greenwich Observatory, already called Greenwich Mean Time. Initially GMT had to be carried by an operative with a clock going on a train. He would stop off at the various stations to correct their clocks. By 1855, the telegraph put him out a job.
There was resistance to London Time. Town Hall clocks and station clocks in some areas gave different times with up to 20 minutes variation. Though, by 1858, 98% of towns had adopted London Time. The final step, which went beyond the railways, was the Definition of Time Act of 1880 which set one time for the whole country.