At what point does a siding become a 2nd main track? And what kind of switches are used? 60 mph? 40? Good old 10 mph hand-throws like SP used to use? One would presume that they're 1) dispatcher-controlled; and 2) fast enough that a substantial train can enter a siding in something less than 1/2 day.

I think there's a functional definition for "long train" - one that won't fit into the sidings along the line, if it's single-track. For those sitting at a grade crossing, it's one that makes them wait for more than 2 or 3 minutes; a 10-minute wait is effectively forever.