Well, even though I'm not a physicist or an aeronautical engineer I have a plausible guess to why the water is going forward. Simply put - some form of turbulence is creating a standing wave type pressure pattern over the railcar. Since the pressure from such a thing is greater to the back of the window than the front... Well, like you said it'd be interesting to model. Too bad I don't know how. I've even seen something similar to this when driving my car. A low-pressure pocket caused by the front roof pillar sticking into the slipstream will suck the rain forward and upward. Also I'm willing to bet if you went to a different spot in the railcar, the rain might be moving in a different direction against the glass.

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.