That was...err...pretty random, Ausir.
They are canon, tho'. I once asked Leonard about them, but he started shaking his cane and going all "gerrof my lawn you damned kid" so I just left it at that. They're only mentioned once, and it's never clear how many the Unity has, nor what they run on.
For those who are not familiar with how energy generation works: all current fusion plants are in fact steam plants, where the fusion reaction is used the same way coal is to transfer the energy (heat) to the water, produce steam which produces electricity. A "steam truck" could in fact be running on fusion, fission, coal or diesel technology, "steam" doesn't say anything about the energy source.