>>Hmmmmm...not sure how ground vibrations would be harmful

>I have an apiary 50' from a busy railroad track. The bees winter fine.

Actually, I have had one that close, too, and not noticed any effect.  I suspect that there is a vibration threshold above which the bees are stimulated to action, and since we know bees have a sense of time, occasional, unpredicable disturbance may be different from constant background vibration. Dunno.

At any rate we can see that certain levels of distrubance will cause the bees to loosen or break cluster, and that is what I imagine to be most harmful, if repeated over time.  

The ambient temperatures could factor in, too, as could be the frequency, amplitude and nature of the vibration.

I don't know how close the snowmobiles were coming to the hives, but if the hives were buried and the machines actually came close or went over, that would be a problem.  

Also, snowmobiles give off fumes that are quite distinctive.  Usually trains don't emit fumes near the hive entrances.

Just some thoughts.  We used to visit our hives occasionally by snowmobile, and that had no effect I could ascertain.

As for the effects of disturbance and the consumption of feed over time, I have updated my chart at http://www.honeybeeworld.com/diary/images/2009/9l1.ht35.jpg for those interested in such things. 

I'm at day 74 of this record and the hives on the scale have each consumed about twenty five pounds of honey.

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