The FP7 was essentially EMD's F7A locomotive extended by four feet to give greater water capacity for the steam generator for heating passenger trains.

While EMD's E-units were successful passenger engines, their A1A-A1A wheel arrangement made them less useful in mountainous terrain.

[citation needed] Several railroads had tried EMD's F3 in passenger service, but there was insufficient water capacity in an A-unit fitted with dynamic brakes.

EMD's solution to the problem was to add the stretched FP7 to its catalog increasing the water storage capacity.

The FP7 and its successor, the FP9, were offshoots of GM-EMD's highly successful F-unit series of cab unit freight diesel locomotives.