Interesting about the sewer. I knew about the Kings Cross effort, but this made me think about some other places where there are awkward infrastructure problems.
The Longford River passes directly under Feltham Station in West London at such a shallow depth, a concrete slab has had to be cast over it and the rails attached directly to it as can be seen in this image.
commons.wikimedia.org
You can see the smooth slab under the front of the train.
Another place is on the Keighley and Worth Valley Railway, where the Bridgehouse Beck runs under the line just south of Haworth Station under bridge 27 as seen here...
The Bridge 27 renewal project is quickly coming over the horizon as the next big civil engineering project for the railway. The planning and design are well underway, with September 2024 being the current target date for the main bridge renewal with a three-week closure planned. However, changes...
kwvr.co.uk
As you can see, there is next to no clearance.
I'm sure that a solution will be found.