Originally Posted by Andy Tejral

I'm not sure but here goes: The more weight you have on the tripod, the more the tripod is going to 'be one with' the ground. The force pulling down firmly connects the tripod to the ground. A lighter tripod is easier to tip over, less connected to the ground.

If you are trying to eliminate wind vibration, that is good--generally the ground doesn't move unless you're in earthquake country, which is essentially what we're talking about here--mini, repetitive earthquakes.. The force of the train literally moves the ground so you want the tripod 'to be separate from' the ground.