Thanks everybody for all the feedback. The highest ISO was 640 with F4.5 (number 6, this stage also had a lot of fire smoke blowing across). They were all taken using AI Servo, evaluative metering, and IS set to panning mode. Most of the others on this page are 200 F 4.5 or 250 F6.3. I don't really see the noise myself. I agree that some are a bit soft. But I wanted to have a bit of variety, so I chose some of those anyway. There was almost always a fine snow dust that came by a while after the cars, but I really doubt that was still lingering around by the time the next one came by.
I know this one came out quite sharper than most of the ones I posted here.
http://www.assortedpixels.net/transportation/rn/b7.jpg
What about input in terms of placement, composition, etc,? Should I have tried some different settings? Is it possible the cars were faster than the camera could compensate for? The only thing I wasn't that happy with was having to redo the white balance (I shot the gray card at the beginning of every stage, but it was almost always off).