A few years back (probably more like a decade now) they closed the London Overground for engineers works over a May Bank Holiday weekend, and opened up the Thames Tunnel for visitors - https://en.wikipedia.org/wiki/Thames_Tunnel - that is to say the original Mark Brunel tunnel from 1843, original a foot passage, but now a part of the London Overground network.

I was lucky enough to attend. It was an interesting experience - from what I recall a lot of the original ornamentation has been lost, but there are some preserved sections.