We did the Amtrak once and it worked out fine for me, but then I'm a night owl. A couple in the group aren't crazy about getting back to our vehicles at midnight. Nothing is going to be as cost effective as Amtrak, (or the Spin Motel for that matter) especially with such a small number of riders but I told them I would at least make inquiries. Again, thanks for your suggestions.
Edit: Another factor is the limited number of bikes on each train, and it seems we may have one or two that will be boxing up their bikes rather than rolling them on.