Do you remember what kind of vehicle it was, or at least where it was?

I think I did already describe a pretty bad experience with a special type of the Siemens Combino for Düsseldorf, which has a leader truck under the first section with the cab. This truck has both, a short wheelbase and small wheels. At about 55 km/h, it started to hunt horribly, and it did not really dampen at higher speeds. The other segments of the Combino did run OK (well, for a Combino).