Don't know about Chatelet but the last time I was to London (which is not more than a year ago), the tunnels and trains seemed fine. Yes, it gets crowded during the morning peak hours in some of the busy routes but apart from that, they seem to be really good. Makes me sad that most other cities do not provide that kind of easily accessible public transport.
Sorry I was talking about Paris. London is a much more pleasant experience, despite the tube being so much older, the tunnels narrower and the temperature very hot in the summer. I avoid taking the metro in Paris, I have no problem taking the tube in London. Not the least because the brits have preserved a form of courtesy without which there would be deads every day in the tube (if people were pushing on overcrowded platforms like they do in Paris).
London's transport system is extremely well run, despite the national governments political hostility towards it.
TfL (Transport for London) do a lot of interesting things to manage traffic (rerouting people on foot through stations in interesting ways) to manage traffic/congestion.
Its also rather heavily surveilled and policed by the British Transport Police (a fully fledged police force specifically for the UK public transport networks).
