by jamoldover
The main reason they've preferred 4-axle locomotives is simple - lower fuel costs. Hauling around the extra weight of additional axles, traction motors, and carbody length when the service you're using the locomotive in doesn't need to have the additional axles, traction motors, and carbody length burns a lot more fuel. That's the same reason why even freight railroads preferred using 4-axle high-horsepower locomotives on hot intermodal trains (examples - Conrail's B40-8 fleet, Santa Fe's GP60's/B40-8W's, etc.).