The temperature ratings on sleeping bags don't mean "snug as a bug" while sleeping in your undies at that temperature. It means you'll be alive, miserable but alive.
What I'm getting at is that even with a cold weather bag you will still need to be bundled up yourself depending on how cold the temperature is and what the rating on the bag is. You lose alot of heat into the ground, lay down layers of cardboard and your bedroll then your sleeping bag. You can wrap yourself in a tarp to help with the wind or with a bit of rope and knots you can make a tent out of a tarp. Either way you'll most likely need a good jacket, bibs and longjohns to get through a NY winter if you're outside with a summer bag. Might want to consider travelling to warmer climate, cracking a squat or even homebumming it and trying to get into a shelter for the winter if it means survival.
If you don't have much money, now might be a good time to fly a sign. Just sayin...
As far as having it take up less space, get yourself a drybag and compression sack and lash it under the pack with the straps. Or even a small duffle bag would do in a pinch. Just don't have a bunch of dangley bits around trains.
Hope that helps.