Why did the development of time zone make train schedule more reliable

Originally introduced as "railway time" or "London time" in England, in 1840, the standardization of time made train travel much more reliable.

Before this every town or city would use its own "local time" based upon a local sundial.  When people travelled slowly by horse and a journey took several days this wasn't a problem, as they'd just change their clock or watch to the new towns "local time".  

Trains are much faster though, and they needed time tables to tell people when they would arrive and leave.  In England this meant that if a timetable said a train left London at 10am and arrived in Oxford at noon London then it would confuse people in Oxford because using their local "Oxford time" the train left London at 9:55am and arrived in Oxford at 11:55am. The problem was even worse in the United States where cities might have local times up to an hour different from each other!

By using a standard time and introducing timezones, starting from the prime meridian in Greenwich, England, 10am in London would be 10am across the country, and in the United States every town in the Eastern timezone had 8am occur at the same time.  This made it possible to print timetables, and made train travel both more reliable and safer!