Compound that with the depth work too.
Unlike most metro systems, many London Underground (tube) lines are bored, and at a much greater depth. The Northern, Victoria, Bakerloo and Jubilee lines go under the river; while most systems route trains over bridges.
The history of the tube is fascinating. The most recent lines (Victoria, Jubilee, CrossRail/Elizabeth) were built by a centralised authority. The older lines were built by various railway companies wanting to extend their lines into London. Over the years, railway companies dissolved and merged, leaving the fairly awkward map (the two branches of the Northern line share a platform at Camden, a stations at Euston and Kennington, and usually nothing else).
Because of the depth and lack of foresight when building anything, changing the network is nigh on impossible without major disruption.
For example, a new terminal is being built at Bank, meaning the Northern Line platform is no longer a ’bridge’ between Bank and Monument. Most of the work is done, but a substantial amount of the line will close for 3 months to finish it off. (Unfortunately, this is my commute. It’s annoying but I’m ok with it.)