On every Monday and Thursday starting in December 1851, William Cranch Bond of the Harvard Observatory checked the mean solar time in Boston (itself set to the Greenwich, England, Prime Meridian) and then telegraphed that time to railroad stations throughout New England. >From those station clocks every train conductor and engineer in the region set their own pocket watches. For the first time, clocks throughout the United States were synchronized.

Before the railroads expanded into regional systems, all land time in the United States was local time. As long as rail systems were self-contained, conductors could run trains by schedules geared to local times posted at terminals. Starting in the 1840s, however, previously isolated train systems began to merge with lines using different local times, which proved dangerous. In 1841 America's first interregional line, the Western Railroad (also called the Worcester and...