The Channel_Tunnel really should have been designed to ultimately have 3 or 4 train tubes. Then each tube could be closed down for maintenance and repairs.
https://en.wikipedia.org/wiki/Channel_Tunnel#Construction Being such a major conduit, there should have been some future capacity included into its design.
https://www.voanews.com/a/eurostar-services-resume-as-cause-of-flooded-tunnel-probed-/7419121.html
https://www.ft.com/content/32a1896a-fc38-4a73-8007-1cf59f9e8e09 Two train tracks is the bare minimum, but 3 or 4 tracks would have been so much better.
https://en.wikipedia.org/wiki/Channel_Tunnel#Illegal_immigration High level security should always be implemented for such crucial infrastructure.
