Whe I was working for the SP/UP railroads, there was no specific "no loner than xxxx"lentght of train rule nor was there any federal regulation regarding such.

The rule(?) most often applied was no tran longer than teh longest siding.

Fer instance: I was working Roseveille,CA/Sparks,NV, we were instructed to make a pickup in Truckee, CA.
The conductor (we still had cabooses) informed the power that be as to what the length of the train woudl be after the pickup.
He(and I) were told to take "all of track #1, the length would be fine."
So, off to Sparks we went five units on the point, a one unit poush behind tghe cabosse 14,000 odd feet of train on the move.

Long story short(end), the train deaprted Sparks eatbound where a non-clearing meeet was made somwhere out in the desert.

The conductor and I heard about it, officials high up asked why did we do that, the conductor coutesously replied "we do what we are told to do and we were told to do make the pcikup. End of story, nobody (at least that I knew of)got fired.

But I bet somebody did.