The basic point of misunderstanding and disagreement on this subject can be perceived by looking at the comments above from Old Timer and Virginian. Virginian has interpreted the regular cycling between the front engine (i.e., front drive wheel set) and the rear engine as a manifestation of slow, regular slippage of the front wheel set. Old timer says no, that “when one engine of an A slipped, it ‘flew up’ and produced quite a different flurry of sound”. The reason Old Timer is right and Virginian is wrong is related to a fundamental principal of physics. When two surfaces are free to move with respect to one another and they are in contact, the movement tends to be prevented, of course, by friction between the two surfaces. But if there is enough force differential between what ever forces are acting on the two different objects (with the two different surfaces), friction will be overcome and the objects will move with respect to one another. That is, the impeding friction is “broken” and the surfaces slip. The important point here, and the one of which most people seem to be unaware, is that there are two different coefficients of friction for the system, the static coefficient and the dynamic coefficient. There is the coefficient when the two sufrfaces are not moving (the static coefficient) and the one when the surfaces are moving relative to one another. The dynamic coefficient is ALWAYS, repeat ALWAYS, less than the static. This means that once the force differential is enough to start movement (and the static coefficient no longer applies) the lesser dynamic coefficient is operable and the surfaces now move more easily than before the static situation was broken. A train drive wheel with a torque on it will therefore speed up greatly as soon as it begins to slip, i.e., when the static coefficient is “broken”. This is why “Old Timer” has to be right.
Of course, one