The BNSF runs a lot more of the trains with DP- even empty grain trains east out of the valley ususally now have one DPU on the rear. This goes a long way to eiminating helper districts and less switch moves. Tehachapi seems to have some DP and tonnage restrictions that are a little more complicated than they were a few years ago, but I can't elaborate as it's not my district.
About amonth ago, however, we picked up an empty grain train out of Trigo, Calif. and when we got to Calwa, we had to set out the head three cars for compliance issues over the hill.
DPUs usually get set up at train origin, thereby relieving places like Bakersfield from getting backed up with all the extra moves.

This doesn't curtail the engineer from using the DP on flat land, although the 'fence' could be put up and the unit put to idle or dynamic brake only. That condition isn't exactly favorable to good train handling, as a locomotive now becomes a huge weight at the end of the train possibly creating large draft and buff forces.