Standard-gauge Ireland, train ferries, and a possible rail tunnel

What changes if Ireland's railways use standard gauge and train ferries cross the Irish Sea?

A train-ferry service becomes plausible and could help justify a later tunnel. An Isle of Man interchange is another possibility if traffic is sufficient.

Matching British and Irish track gauges would make an Irish Sea train-ferry service plausible, although such a service would not necessarily survive to the present. Ferry traffic could strengthen the case for a fixed rail link, as ferry evidence did for the Channel Tunnel. An Anglo-Irish tunnel is consequently suggested as a possible contemporary of the Channel Tunnel in this counterfactual. Standard gauge on both sides could also change the case for the Isle of Man's three-foot-gauge railway. With sufficient traffic, the island might adopt standard gauge and become a central interchange for ferries between Britain and Ireland. That role depends on traffic volume rather than gauge compatibility alone.