I've found it very interesting too, particularly some of the little known items like the never-completed station under Hampstead Heath. Nice to see understandable descriptions of the engineering involved, rather than focusing on argumentative passengers and the like, as some earlier series have. It also seems to have avoided the portentous mock jeopardy of too many Ch 5 documentaries (" ... If all these signalling systems fail, then trains could crash into each other and HUNDREDS of people could be killed ..." - err, yes, that's why we DO have those signalling systems).
So far a good series, and I hope Ch 5 will continue and cover each line in time.