The best bet is a cripple being returned to a shop or post shop, back to the mine or port. They are used for scrap tie loading nowadays, but in 1973, they'd have been way too new. Then there is the mis-routed car, though kinda hard to do on a unit train, but humans will be humans...
Unit train size depends on the size of your layout. Given your plan, 20 cars should suffice.