Before the railways (railroads) came, there was no particular reason why people in Bristol, England should keep the same time as people in London. At that time there was no practical way of communicating information about time over a distance. When the telegraph made such communication possible, it became necessary for people living in one area to agree that they would not keep their own local time, but would all keep a time based on the local standard meridian. Bristol is at 2º 35' W(est) of Greenwich, so when it is noon in Bristol is just past 10 past noon (twelve) in London.
There is still a relic of this change; the clock over the old Corn Exchange in Bristol has two minute hands. The black minute hand shows Greenwich Mean Time and the red minute hand shows Bristol time!