I commute by train from NYC to the suburbs (northeast corridor), use EVDO card on the train. What I find is that as the train is moving, connection will be maintained, but latency will change, at more or less the same points on the route, from ~100 ms to 500-1000 ms, more or less instantly. Once the latency goes up, it jitters in the high range and never comes back down to 100 ms. Interestingly enough, if I kill the connection and re-establish it, the latency goes back to 100 ms and stays there (until I reach the next step-up point along the route). This is true both under Linux and Windows.
There are a couple of step-up points, all of which seem to be located in NJ and between Newark and the Hudson River. West of Newark, its smooth sailing for about half-hour.
I assume that increased latency has to do with hand-off between towers, but I am very puzzled as to why it would go up by so much. An extra IP hop should not be that costly. Perhaps during handoff, a modem is allocated the least possible amount of bandwidth?
Low latency is important to me, as I run interactive applications (over VNC or SSH), and keystroke latency at 100 ms is very usable, while at 500 ms its pretty much unusable.