One day when I was watching a freight train go by, I noticed that all the diesel-electric locomotives had the wheel assemblies with six wheels and three axles at both ends. I have seen diesel-electric locomotives that have wheel assemblies with four wheels and two axles at both ends . . . what is the utility between those two kinds of locomotives? What is a four-axle diesel locomotive good for versus its six-axle cousin? Why can we not have a five axle diesel-electric locomotive (a three-axle truck in the front and a two-axle truck in the rear)?
Six axle locomotives ride much better than four axle ones.They also pull better and have better wheel slip control due to the fact that the power output is spread out into six traction motors instead of four.Their drawback is that they wear the rail faster on curves(they are trying to find answers to that with things like steerable trucks that have linkage assemblies to help guide the axles around curves).Four axle units have shorter trucks that can negotiate sharper curves.On many industry tracks four axle locomotives are the only engines allowed to use the tracks.For that reason many road switchers use four axle locomotives.
The two-axle trucks (called B-trucks) are lighter and cheaper to produce, but they also have less power, having only 4 traction motors instead of 6. Most early diesels had B-trucks, but the need for more power means that nearly all electric and diesel-electric locos now have the 3-axle trucks, called C-trucks. There was a 5-axled locomotive, with 2 axles on the front truck and 3 on the rear. It was the EMD FL-9, produced originally for the New York, New Haven and Hartford. I think they eventually ended their working lives working for ConnDOT. The rear truck was an A-truck i.e.the two outer axles were powered but the middle one was an idler, used to spread the weight and give a lower axle loading. These locomotives were referred to as electro-diesels (as opposed to diesel-electric) because they could be run as a pure electric locomotive using a 3rd-rail pickup. Edit: Thanks Damon. The way you can usually tell is that the middle axle might have smaller bearing boxes because it doesn't have to deal with the weight of a traction motor. But that's not always the case. Good on you for thinking outside the box. Another Steve Jobs.?
In basic terms, it all comes down to weight - the heavier the locomotive is, the more wheels it needs to support it. Like roads, bridges and so on, railroad tracks will only support a certain axle-loading. The size of a locomotive is related to its power output, due to having a larger engine and generator, and thus the more axles it has, the more powerful it is likely to be. However, if the track that it has to run over can only support lighter axle-loads, then the locomotives will have to have more axles to spread the weight. Here in the UK, some of the early diesel-electric locomotives of the 1950s had additional un-powered axles, as these locos were heavy for their power output. Thus, the earliest locos in the 2000-2500 hp range had two four-axle trucks, the leading axle in each truck being un-powered, with motors on the other three (1Co-Co-1 arrangement). The medium-powered locomotives built by Brush had two three-axle trucks, on each of which the middle axle was un-powered (A1A-A1A wheel arrangement). EDIT: I've never seen the point of proposals for nuclear-powered locomotives. What you would in effect be building is a steam locomotive with a nuclear pile inside the firebox! It would be extremely large and heavy for its power output, due to the shielding for the reactor, and the additional weight of the auxilliary steam circuit, heat-exchanger and its condenser. If you are going for direct-drive then you have to use conventional reciprocating pistons, which are not that efficient, or a turbine, which is only efficient if run at maximum output continuously. If you opt for electrical drive you are adding even more mass, due to the turbine, generator/alternator and traction motors, let alone the electrical control gear.
