How the railways set the clocks
Before railways, noon was the moment the sun stood highest over your town. Because the sun reaches a place further west a little later, Bristol clocks ran about ten minutes behind those of London. For stagecoaches this hardly mattered. For a train that left at a printed minute and had to be met by another on the same single line, it mattered a great deal.
The Great Western Railway was among the first to adopt London time at all its stations, in 1840, and other companies followed over the next years. Passengers travelling west found that the station clock and the clock of the town square disagreed, and some towns showed two minute hands, one for local time and one for railway time. In 1847 the Railway Clearing House recommended that all British companies use Greenwich time, and by the 1850s most did. It was only in 1880 that the law caught up and made Greenwich Mean Time the legal standard in Great Britain.
Time zones in America
In North America the problem was larger. Railways ran their own local times, and the timetables of different companies were hard to reconcile, with dozens of local standards in use. On 18 November 1883, at noon by each region's local time, the companies introduced four standard time zones across the continent. The day is sometimes called the “day of two noons” because clocks in many places had to be set back. The government did not give the zones legal force until 1918.