Northbound coal trains south of Binghamton are assigned helpers to East Binghamton yard.
Any other trains only get helpers when they stall, and those units are off the nearest train to shove. Say for instance, a 250 stalls north of Tunnel, if there was say, a 169 waiting at CPF-590 in Afton, they would cut their power, come up the hill behind 250, tie on to the rear, and shove 250 over. They would then get back on their train, and continue on their way.
Last year, a 252 was split in 2, with power from a 930 added on. They brought the intermodal block down to Binghamton first, then went back up to Afton, and grabbed all the freight.
If something stalls Northbound out of Binghamton, they may just send up spare power from the yard. Or yet again, power of a train behind it.
This applies to all hills, Belden, Richmondville, Clarks Summit, Yatesville, etc.