One reason the South lost the Civil War was that there was no standard "gauge" (the distance from the left hand rail to the right hand rail). It had been mostly standardized in the Northern states.
When standardized, trains can run from state to state - and freight and passengers do not need to be unloaded and reloaded at different cities.
To FINALLY get the nation onto a standard gauge, there was a requirement to stop all traffic and move one side of the rails.
The date chosen for this monumental task was