I just got a Dahon Mariner D8 a couple months ago. It has been a real nice little bike, considering what it is, so far. I have some changes to do to it to suit me a bit better, but the whole stock bike as a starting platform is awesome. I got it as a compact bike to go where my big bike can't so I have something to ride more often. However, for my trip coming up it isn't going to work at all. I need my disk trucker and all my gear as it will be a long trip (for me) and not something the Mariner is cut out for.
I saw the folding bike accommodations on Amtrak's website. That will be good to know in the future if I do a lighter/smaller trip some time where I can get by with the Mariner. However, I think I want to keep the heavy riding to the Disk Trucker - that's what it is made for and why I got it. Plus it has a ton of gearing for anything I could throw at it.