[sjsamson]

There are overhead electric rail lines supporting double stack freight in the NEC USA, China, India, and possibly elsewhere. There are YouTube videos and images on web. AAR Plate H (20’ 2”) is the North American rail loading gauge for double stack containers, so 21’-23’ for the electrical lines is enough clearance. Nothing unusual here, straight forward for new lines, but as you noted potentially a lot work/cost on existing lines to create enough clearance due to raising overpasses/structures and/or undercutting them and tunnels.

[Animats]

That's tight. Here's a document from Network Rail on the practical problems of trying to increase the loading gauge in the UK so containers can fit on electrified lines.[1] That's just to make room for single height containers. In the UK, the catenary clearance is sometimes reduced to get under historic bridges, but that requires slowing trains way down. Sometimes the rail bed is lowered, but that creates drainage problems. Raising historic masonry bridges is even harder. This document has good pictures of all the problems.

[1] https://www.bathnes.gov.uk/sites/default/files/sitedocuments...