The GP40 is 3 feet (0.914m) longer than its EMD 567D3A-engined predecessor, the GP35, and distinguished visually by its three 48-inch radiator fans at the rear of the long hood, while the GP35 has two large fans and a smaller one in between. It was built on a 55ft (16.76m) frame; the GP35 was built on a 52ft (15.85m) frame - as was the GP7, 9, 18, and 30. The difference in length can be seen in the GP40's ten handrail stanchions compared to the GP35's nine.
Union Pacific has rebuilt 129 of their GP40's and GP40-2's into GP40N's at their Jenk's shop. These units received a microprocessor control system to increase adhesion, control options, and extend the life of the locomotive.
GP40R
The St. Louis Southwestern Railway (Cotton Belt) ordered eight locomotives from EMD but rebuilt all eight of them into GP40R's between January and June of 1982.
The GP22ECO is a locomotive built from recycled GP40/GP40-2 parts. The frame, trucks, and carbody are refurbished, while the 16-645 engine is replaced with a new 8-710-ECO engine. The electrical system is replaced with a modern microprocessor-based system. Horsepower is reduced to 2,150 due to the smaller engine.
Penn Central #3118 is operational at the Austin Steam Train Association (ASTA) as ATCX #3118. The locomotive currently wears a blue paint scheme from its previous owner (Horizon Rail). It has also been updated with a microprocessor, making it a GP40-3.