When the GP40-2's showed up they were chained together in consecutive numbered sets. The idea was to keep them on the premium intermodal trains and to take advantage of their "Select-A-Power" feature that could place trailing locomotives in idle to conserve fuel. I remember one of the units had a mechanical problem and was set out at El Paso and used in local service until its set returned from the east.
About the same time SP hired McKinsey and Company to create protocols to use the fleet more efficiently. For example, since Los Angeles had more traffic flowing to it than away, trains to the Basin were to operate at strict horsepower per ton guidelines - something like 1 HPT for merchandise and 3.5 for intermodal. Trains regularly had additional units added to send power back to protect critical points like Pine Bluff. The plan staggered on for awhile and the Power Bureau frequently sought out "hidden" locomotives like an extra helper set at West Colton that someone was keeping "just in case."

Here is some information on Select A Power. A mechanical person told me the GP40-2's exhibited uneven engine wear after about a year with intermediate units being regularly placed in idle.


Patent