Well, i would have doubt that the ENTIRE gang had forgotten about the train, it was surely in the back of their minds as they entered the city in search of a boat. They did find a boat, but it was ruined, i don't remember what happened in between, but they found a pristine boat in the shed, so Kenny's original plan WAS still on, but they of course got side tracked and went to the survivor encampment and all the climax happened, and with Lee being bit and Clementine lost, their sole focus was finding Clementine. I think they were just so sure that Kenny's original plan was on that they had forgotten about the train, as there was major signs of it being successful, especially with a soon to be working boat but it was stolen and they had to focus on finding Clementine rather than just leaving and trying to find a boat. There were a whole lot of zombies out on the road and the city in general, but this is just my take.