My first impression of Big Train when I saw it all those years ago was that the staring competition was fabulous but the rest of it was average. It had good ideas but they just went on too long.
Having seen some clips on Youtube, I feel the same way. The wank scene for instance: good idea, good Simon Pegg and others arguing about it at first, good when the guy chips in opposed it and good at the end where a man is still at it. But Simon Pegg's argument goes on too long for me, and Amelia Bullimore is not really needed. She doesn't add anything (does she ever?)
Same with the sketch where the woman wants to know if Simon Pegg speaks English and he says 'no' in English. Another good idea, but it goes on too long.
Same with the 'Tom Robinson' joke and the one where Kevin Eldon thinks he'll use somebody's opinion next time he's out. They're like 20-40 second sketches dragged out for minutes.