About the Santa Fe PA/PB

The Santa Fe used ALCo PA/PB units for passenger service, all wearing the famous "warbonnet" paint scheme. They have pulled everything from the San Diegan to the Super Chief. The ALCO PA/PB units are easily discerned from other E and F units by their distinctive long, rounded noses.

A and B Units:
In railroad Terminology, an A unit is the more commonly understood locomotive design, featuring a crew cab. B units lack a crew compartment and therefore are driven from a coupled locomotive with a cab. B units were often referred to as "booster" units, as their purpose was to supplement the pulling power of an A unit.