49 CFR 229.119 Cabs, floors, and passageways (Cab Heat)

(d) Any occupied locomotive cab shall be provided with proper ventilation and with a heating arrangement that maintains a temperature of at least 60 degrees Fahrenheit 6 inches above the center of each seat in the cab compartment.

Maintains is the key word in this. Temp must be at least 60 deg. F 6 inches above the center of each seat in the cab even when moving.