The Ft units from EMD are diferentiated from later f units by a couple of spotting features. They don't have symetrical truck spacing relative to the body like other f units. There is extra overhang of the body at the rear of a units or one end (front) of the b units. Originally they were intended to be operated in pairs of an a-b set and the a & b units were permanently connected via draw bars. Santa Fe, and I think most other railroads that had them, replaced the draw bars with couplers to give them more flexibility. Other distinct differences: They have 4 "prothole" windows on each side of the a units. The B units have 5 portholes on one side and I think 4 on the other side. The a units have 3 tall rectangular number board windows in the center of each side in addition to the small number boards on the side of the nose, no number boards on the b units. Dynamic brakes, where used are large rectangular boxes with a flat top sticking out of a curved roof.