I suspect that the tolerance of track and wheels, with the addition of the need to splay somewhat on sharp curves where long wheelbased non-bogie vehicles are allowed, leads to some quite generous / sloppy motion at low speeds. A pulled vehicle with appropriate rail and wheel profiles will tend to centre, a pushed one has different forces so may go "to the side" somewhat, and all will be further influenced by the coupling type, braking forces, and whether the vehicle itself has equal break forces to those either side of it, or less or more brake effect. You then need to consider rail grinding at maintainaince time, and the gauge widening effects of running vehicles of certain types on certain curvatures; preseumably the effect of the rails would differ with "push" or "pull" and in one direction would probably put different wear on the track to the other - so the risk might be of track damage rather than / in addition to vehicles thumping the platform, or something on the other side.
It's too complex a subject for me to get my head round in the few minutes I have. Here are some starters for those of you with more brainpower, more time, or more background knowledge of this sort of thing:
http://en.wikipedia.org/wiki/Standard_gaugehttp://wapedia.mobi/en/Rail_gauge http://www.freepatentsonline.com/4386469.htmlhttp://www.encyclopedia.com/doc/1G1-15486422.htmlAs I read it, adding track and vehicle tolerances you have a "play" at rail / wheel level of about 40mm on straight (60 m.p.h.) track ... and that's before you even go on to consider any sponginess / rockyness in the joints. I remember how the Hastings units used to rock around, and I seem to recall they had rubber bushes as limiters. Then there are end of vehicle overhang issues and splaying on curves.
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