About the Great Northern F7A/B
The Great Northern Railway, running a distance of more than 1700 miles from St. Paul, Minnesota to Seattle, Washington, was a privately funded transcontinental railroad developed from the St. Paul and Pacific Railroad. F7 Class locomotives, when pulling Great Northern trains, would run in an A-B-A formation with the three locomotives sharing the same number but differentiating from each other by either an "A", "B", or "C" designation.
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.