Thanks for all the informative responses. Some replies and follow-ups:
How tall am I? 6' even, with 32" inseam. Sounds like most bikes should be generally a good fit.
SteveDahon: I would switch to Amtrak with the folder, rather than primarily SEPTA. Sounds like they should still accept 20" folders with a cover.
Chainguards are probably very important.
The 2007 Dahon Ciao P8 and the Dahon Mu XL are looking like top contenders, with no real flaws as I can tell.
The Ciao/Vitesse P5 doesn't have low enough gearing for me. The Mezzo bike looks interesting but I didn't like the gear range on the 4 spd hub. The DowntubeVIIIH lacks chainguards and front fender, otherwise would be good (though would need to add lighting). Birdy looks nice but once again not too much in fenders or chainguard, and fairly expensive.