S1·E5 Thomas's Train

Thomas wants to pull a train instead of just shunting coaches for the big engines, who laugh and say he wouldn't be able to do it. One night Henry is ill, and next morning he still isn't better. He is due to pull the morning train, and when no other engine is available, Thomas has to pull the train. He is so excited, he doesn't notice until too late that he has started before being coupled to the train, leaving his coaches and passengers left stranded at the platform!