As SP Fan noted, it was common to find Espee freight units on the point of the SFZ over Donner Pass during bad weather, as their larger plows and extra horsepower were useful in snow fighting. Remember too that Espee requirements for icicle breakers found Amtrak installing them on second - hand EwePee E - Units in the early days. I also wonder if perhaps, back in the days when the host RR provided the engine crew, if the RR was more comfortable providing a home - road lead unit as well ??? As noted by others, Espee had only a little trackage with passenger speeds over 70 MPH.
The issues with Amtrak's SDP40F locomotives causing derailments were the reason BN restricted them, then banned them outright. Strange thing though, the Santa Fe, which ran the SDP's @ 90 MPH, placed no such ban or restriction on the units.