"The" reason seems implausible given even back then you would have an officer per 500 miles, which doesn't seem useful.

You don’t need the same level of coverage on the thousands of miles of track in deserted rural areas as you do where the trains are frequently stopped in urban areas where millions of people live. As trains have been getting longer to reduce staffing costs they have been leaving them on the tracks rather than in fenced in yards, so those specific areas are where they need security guards.