The south siding at Cumbres will easily hold 50 cars. I have an employees timetable which shows Cumbres at a total capacity of 105 cars. As another note, I have train register sheets showing 51-car 3-engine hill turns of empty pipe gons and idlers.
Backing the two sections together with one engine would be tough. The engines don't get a good grip on the rail shoving backwards. Some of the engines did not have back up sanders. If they did, they would have been behind the #3 drivers, leaving the #4 driver unsanded.
I ought to ask one of the guys who did it all the time and see if there was a standard routine for setting cuts out at Cumbres.