SD40-2

Patrick,

The basic differences come from the type of components used on the locomotives, based on when they were built. If you look at the radiator intake grilles (at the rear of the locomotive), you'll find one of the following variations on the standard SD40-2 model:

1972-1976 production: 81" long short hood, with wire radiator grilles (grille will have a repeating squarish pattern from where the wires crisscross each other). 1976-1984 production: 88" short hood, radiators intakes consisting of a metal mesh screen pressed into vertical corrugations. Very often referred to as "louvers" by their appearance.

The "Q" fan model looks very similar to the 1976-1984 production, with the prime differences being the type of radiator fans (the group of three fans at the rear), exhaust hatch, and dynamic brake blister used. The "Q" radiator fans have a slight dome shape to them, while the others are flat. On a "Q" model, the exhaust stack is also contained in a raised silencer housing, while the stack on a standard version is raised from the flatter mounting "hatch." "Q" models also have a dynamic brake blister that bulges slightly wider at the front, whereas the other models are a fairly consistent width.

It should be noted that "Q" versions were only produced after 1980, to comply with federal noise emission standards.

You might also find other variations of the SD40-2, such as longer nose lengths on the "snoot" version (which often housed radio control gear), late production models with different traction motor blower housings, and units with modified radiator intakes for operation in areas with lots of tunnels (the SD40T-2, or "Tunnel Motor").

The above should pretty much cover the basics...

Thanks for the information.