Re: Double Heading

The second engine, the road engine assignment, is based on power requirements for the total route. The helper is added as required for a specific hill, valley, crater
or whatever where some additional power is required. SP ran Moguls ahead of Cab Forwards on occasion, the helper locomotive rarely was in a situation where it
had to run at full road speed hence 2-10-2s helping UP passenger trains up Cajon Pass. Few exceptions like the Mail on the SP demanded two cab forwards and
both were capable of road speed. In the UP case it would be possible, though unlikely, that something as large as a Big Boy would help a Northern powered train
up out of Cheyenne though it could happen if the Big Boy was required for helper duty out of Laramie and was returning.