For me, traveling with a 20 pound baby was much too heavy for my baby Bjorn...
What DID work for us was to put the baby (8 mo/20 pounds) and his sister (2 years/30 pounds) into their own cheap-o umbrella strollers ($20 at Babies R' Us). We folded them up on the trains/subways if there wasn't room for them to be open with the kids strapped in. It only took a second to step to the side, pop the kid out and fold up the stroller to carry it up the stairs once we reached our stop. We did an entire NYC/Philly/DC road trip that way. The umbrella is so light, it's really easy to carry the baby in one arm and the umbrella in the other, and just go!
I hope that helps!! I did try to do our road trip with the carrier...but after the first day, I was POOPED.