02-01-2010, 09:59 PM
I believe the six axle indication refers to diesels that have two 3-axle trucks. 3 axle trucks have a very short distance between the axle centers in the trucks, thus making it more difficult to run through smaller radius curves. This applies also to three axle trucks on passenger coaches. Larger steam engines, particularly with three or four drivers will require that the center driver(s) be "blind", that is, not having flanges that would otherwise jump off the track.
Gus (LC&P).