Dingler refers to the well-known example of the accelerating railway car. Although by now this example has become rather trivial as a result of its frequent discussion and was completely clarified by relativity theorists, particularly in connection with Lenard's repeatedly proposed misunderstanding, we will consider it again here for the sake of completeness. Dingler explains that the braking railway car would not be comparable to one in uniform motion because in the first case the (negative) acceleration does not directly act on the objects in the car, but rather will be first transferred to them by elastic forces. Only in a “real” gravitational field, e.g., one that is parallel to the rails and brought about by a single large mass, can we have complete equivalence; but here again there is no perceptible difference between uniform and accelerated motion if we discount frictional forces. The latter is true, but the standpoint of Newtonian theory does not provide an adequate explanation of it because the accelerated train in this example represents a system (in free fall), for which the Newtonian laws apply, although it itself experiences an acceleration relative to the collection of uniformly moving inertial systems of the cosmos.

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