I know that this is about the best locomotive, but I believe that we need to look at not the locomotive, but the features that they featured. Here's some examples:
1. The steam locomotive. Now it requires only one locomotive to haul the same amount of stuff that used to take several horses to haul the same amount of stuff.
2. The knuckle coupler. Now a person doesn't have to stand in-between two cars that use link-and-pin couplers to guide the link into place to put the pin in place to keep the train together. Standing in this place would sometimes result in the loss of body parts.
3. Superheating. Steam locomotives with superheaters could now produce more power than a locomotive without a superheater. This theroretically means that a 2-8-0 with a superheater can pull a longer train than the same 2-8-0 without a superheater.
4. The diesel locomotive. Now railroads didn't have to worry about large coal towers and water towers placed along the tracks to fill up the empty tenders. Plus the steam locomotives was built for a specific purpose, while, with only changing the gear ratio (and a steam generator) will allow a diesel locomotive to haul both fast passenger trains and slower freight trains.
5. Dash 2 electronics. Without this, locomotives such as the SD40-2 would not have happen.
6. The HTC truck on the SD40-2. Now the traction motors will be set facing the same direction, removing the problem of backwards-facing traction motors causing a rough ride from the torque of the backwards-facing traction motor rotating the opposite direction.
7. The Radial Truck. Now larger locomotives can handle curves that locomotives the same size without radial trucks will just derail on (i.e. Alaska Railroad's SD70MACs are the only 6-axle locomotives on the roster, which is mostly Geeps, because their radial trucks can handle the curves that, before the 70MAC's, only the Geeps could handle)
8. A.C. Traction. A coal train that would require 5 SD40-2's, which use D.C. traction motors, now only needs 3 A.C. traction locomotives.
9. Gensets. The most advanced, enviromently-friendly locomotives produced (behind electric locomotives)
10. Microprocessor control. Now a computer can 'keep tab' of everything going on on the locomotive (i.e. the amount of fuel being comsumed, the amount of air in the brake cylinders, etc.).
Anyone know of anything else?