Gbm,
thank you for pointing out this problem. Coincidentally, I have been working on implementing full weight limits in the next release of Pak128.Britain-Ex, so the issue with tunnels will be fixed then (there will also be new way and bridge types, although, for the time being, re-using existing graphics).
You cannot change the simuconf.tab settings retrospectively for an existing saved game, but when the new version of Pak128.Britain-Ex is released, those changes will apply retrospectively to an existing saved game.