Prior to passage of Uniform Daylight Savings Time some time in the '50s or 60's, I believe, the dates of the start and end of Daylight Time could be set arbitrarily in different areas. You can imagine the confusion that would have occurred in rail timekeeping and timetable publishing if each affected section of route tried to follow local changes as they occurred over a period of several weeks, or not at all. Hence the observance of Standard Time year-round for rail operations.

Once every state had to observe the same dates for changing the clocks if they chose to observe Daylight time it became reasonable for the rails to adopt Daylight time between the given dates across the system and nation. As noted in Amtrak timetables, for example, Arizona and Indiana have chosen not to observe Daylight time at all, which does not pose a problem for scheduling. I was young when the Uniform act was passed, but well remember the days of "Pacific Standard Time" during the summers at the local SP passenger depot.

Year-round Daylight time was tried one year in the '70s for "energy conservation" and was a disaster during the dead of winter. Later, the national dates were moved from Last Sunday in April through First Sunday in October to First Sunday in April through Last Sunday in October. Now the politicians are fooling with it this year two weeks at a time on the theory that people will spend more money if they have more afternoon daylight. What once was done for good reason seems to have degraded into politics for no reason. Clarification is welcome if anyone has the exact years of the changes.