That's us watching it over and over!It is in 'most popular' on Iplayer!
Seriously, nice as it might be to make another series - the fact is that not a lot would change. Joe Public wouldn't care to see what's essentially the same, from similar engineering work to the introduction of what are essentially the same trains on the Circle, H&C etc.
I suppose there might be some scope to follow BTP around, in the same style as all the other police/emergency services shows. I reckon it wouldn't show the BTP in the best light though, as you'd just hear officers saying 'ah, sod that - that's a 5 minute walk away' and having another cup of tea. [JOKE!]