Public transport options from DC to Princeton, NJ? (Washington, D.C.)

Hello

I'm a member of the Vienna, Austria, group, and I'll soon head to Canada and the U.S. for vacation. I'm looking for public transportation options from DC to Princeton, NJ - is there something in addition to the two early evening Amtraks that would be convenient? I'd prefer not to fly - I'll pollute enough during other segments of this journey.

I don't want to decide the exact travel time and day ahead of time, but it's likely to be around 16 September.

Would be grateful for advice. If anyone travels to Austria or Sweden (my native country) I could offer some advice in return!

Schöne Grüsse aus Wien!
Susanna