Probably weight of train or destination or terminal or routing of train, keeping units as one “locomotive”, need of power for use at destination, power move, there really isn’t one answer to the general question but one answer to the single one time situation.
All these “reasons” about balancing power or the happenstance of seeing the train between leaving off hundreds of tons at the previous stop or needing to pick up more tonnage at the next yard are fine theories… but I think the crew had the choice of either riding in an old rattle trap GP-40 or a brand new ES44AC and they simply opted for the comfort and status of the bigger brute.
Couple - three years ago I saw and posted here about an NS switching crew delivering a single ‘hot’ car - a covered hopper of cocoa beans or powder that was urgently needed at a local chocolate plant - to the serving shortline, the East Penn Railroad at Perkiomen Jct. just east of Emmaus, PA. That crew was using a pair of NS Dash 9’s . . .
- Paul North.
Maybe they needed an extra locomotive somewhere, and it cost less to have 1 crew run two locomotives then have 2 crews operate the two locomotive. They might have had a large train that they were going to pick up, or just dropped a big train off.
99% of the time crews have no choice of power - it is assigned to the train by the ‘power people’ (different railroads manage their power in different ways), or the Yardmaster tells each yard crew the power they will use for their tour of duty. Only changes are when power is shopped.