ISP is a bad bad bad idea. Way too far, way too much of a hassle, and even then, it puts you at BWI
, where you have to then get into DC, another hassle!
If you want to do it cheap: Greyhound or Fung-Wa (Chinatown Bus). I know the Chinatown bus is only ~$20 each way and will go direct to DC. Greyhound has been fare-matching and shouldn't be much more. Ride will take ~5 hours. They'll also go from city to city -- no commutes in from airports -- so quite a good option really.
If you want to do it for comfort: Amtrak -- either a regional train or Acela. Regional trains are just fine for comfort -- they'll just make a few more stops and take longer. Acela is very nice. They are very comfortable and a great ride, but will cost you almost 2x the regional train. Check prices and schedules at amtrak.com. Note the train won't be that much faster than a bus -- although not affected by traffic, and the Acela doesn't really go much faster than a regional train, it just makes fewer stops. The trains will go Penn Station to Union Station -- midtown Manhattan to downtown Washington -- definitely the best way to go if you spare the cash.
If you REALLY REALLY want to fly: Take the US or DL
shuttle from LGA
. It'll cost about the same as the Acela, but you'll have to get out to LGA
and into DC from DCA
(not cheap via cabs!). I don't recommend it over the train, but it is definitely faster.
Don't, under any circumstances, take WN
out of ISP
. It will take longer door to door, cost more, and make you feel even more like cattle than Greyhound.
Since people asked about the Acela -- I've taken it many many times between Philli, NY, and Boston. It is wonderfully comfortable -- seats are nice with good leg room. Bathrooms are even nice. Cafe car food is crap, but snacks and drinks are great. It is just as nice, if not nicer, than the TGV in France, Thalys in Benelux, and the ICE in Germany (all of which I've been on). HOWEVER, it does not go nearly as fast! It only goes slightly faster than a regular Amtrak train, its just they are more express. This is primarily because of the tracks. So, I highly recommend it, but don't expect speed. It's still a 3+ hour ride Boston-NYC, while the flight is only 35 minutes. I used to take it from Wilmington, DE
to Boston (a 5.5 hour train trip!) over flying PHL
when the weather was bad and I knew I'd be delayed at PHL
for 2-3+ hours. Even though the train may take a bit longer, I could work and be productive, not have to worry about what time I'd get in, and just enjoy the ride, instead of being stressed at the airport (or on the plane sitting on the tarmac, as is usual at PHL