A 6 episode series that could have been wrapped up in 1 or 2.
I'm an IT consultant of 20yrs and never have high expectations any time cyber stuff is used as a plot line. However on this I do give the BBC some degree of credit as they used convincing phrases and whatnot that do have parity in real life. Where this was let down however was the various twists and turns, i.e. the stowaway on the train, that became annoying after a while. The lack of any attempt to decouple the engine from the carriages was also annoying so I was happy to see this finally attempted later in the series albeit the old fart forgetting his watch and taking a bunch of other characters with him to the front end of the train.
Casting wasn't great, nor was the dialogue (the cheesy jokes didn't add any kind of realism to this). And the ending was a let down... rather than the wrath of going to a journalist and breaching the official secrets act dude went full 'bad guy' and somehow masterminded the puppeteering of various other characters in what could have been achieved by a simple whistleblowing (excuse the train related pun).
The BBC harped on about this being a fantastic show, but in my opinion it was more middle of the road (or railway track?). Worth a watch in the background but there's plenty of better shows out there.