The train stretched my suspension of disbelief from the start. Aside from the ridiculius length/doors etc, where were the police?
BTP are probably one of the most overstretched forces in the UK. Whilst a lone copper in a car could have to cover 200 square miles (at its very worst, but admittedly in a more sparsely populated area), BTP officers have to cover thousands of miles of tracks and stations.I think he means the British Transport Police - since the terror attacks there's often a BTP copper on a train.
These days, you are lucky to see BTP at Waterloo Concourse, let alone on a train anywhere. They have become completely reliant upon the train guards and drivers calling incidents in and desperately attempting to reach the train at the next station in a half decent time (current expected response time to an immediate incident is 20 minutes). I've not watched the show so I've no idea where the train incident occurred, but most BTP officers are based at large hubs and attempt to respond from there. I do not envy them.
Not seeing a BTP officer on a train is probably one of the most believable things Dr Who has done.
Hugo - Coppers on a train? As likely as Snakes on a Plane