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It could also be that I'm at ISO6400, 1/800th @ f3.2, the train is moving, and I'm just pushing the envelope too far.
Yep, ISO 6400 is useable for fairly even lighting, sports in ice rink, but will suffer badly in high contrast situations. Dynamic range contracts with each stop of higher ISO. For my taste, ISO 1250 is my limit and even that needs careful NR. My fav high ISO on the 7D is 320 and 640.