Since you're using SEPTA trains, I would go with a 20' inch wheel folder. I'm going to go off the chart here and be different because that hill at 35 mph would scare the daylights out of me if I had to go down it each month especially in the rain. I would really look at a folder with disk brakes or you're going to wear out a lot of brake pads each day. Unfortunately, there aren't many folders out there with disk brakes but I'm sure Bike Friday will make a custom job for you. Dahon makes the Matrix with disk brakes but that's a 26' inch wheel bicycle but I'm sure SEPTA won't give you a hassle if it's folded and it a bag. They will require the bike to sit in the vestibule regardless.
The other choice is the Dahon CIAO P5. This bike has everything you wanted in your list including dynamo lights (front and rear), rack and chainguard which I consider VERY IMPORTANT for the commuter. I stained a lot of pants with my old Presto back when it didn't have a chainguard. In addition, when bagged, it will be accepted by SEPTA and AMTRAK but there's more.
I think the Ciao comes with a rear hub coaster brake which is a major plus for that big down hill at 35 mph. I have become a huge fan of the rear coaster brake since acquiring a Dahon Presto that has one built into the rear wheel. I'm able to coaster brake almost all my stops and hardly ever use the FRONT brake except for emergencies. I know there is some fade on a coaster brake (there will be some at the end of that 35 mph hill) but it's never a concern and I'm always able to lock up the rear tire if need be. You can always use the front brake if need be but most of the time, the coaster brake will be enough. In short, the coaster brake is low maintenance and will preserve your front brake.
The only thing I don't like about the CIAO P5 is the handlebar because it's fixed and maybe too high if you're under 5'10. Otherwise, this is my choice.
OH. One more thing. I have the Dahon Vitesse P5 and it uses the same rear hub without a rear coaster brake. ;-( I found the Vitesse to be too high geared in direct drive so I fixed this by purchasing a larger cog (Sturmey Archer compatible - Thanks Sheldon) and now the bike is perfect. Some people feel the gearing is fine.
Last edited by Dahon.Steve; 01-14-07 at 01:13 AM.