It's been so long since I've done STP I can't say I remember when we made the reservations.... Probably a couple months in advance, whenever we made our hotel reservations. It's nearly impossible to get a bike rack even though some car rental agencies say they are available, but we managed just by using the trunk and back seat. I don't think too many people actually go the car rental route, so I don't think its too hard to get a car. We always stayed and went back on Monday, so that may have made it easier too, but it looks like you are doing that as well.

You might ask around on the Cascade board to find folks who've used the train. From what I understand the bike accommodations are pretty good and the bikes are safe from damage etc. (much better than an airline - you might even be able to rack your bike yourself) I think there are hooks on the baggage car wall to hang the bikes from. There's only a few spots though and if you don't get them you have to box the bike and check it as baggage which is of course a pain and might even cost extra. It's not really worth it then.