The main point of this PR (https://github.com/jamespetts/simutrans-pak128.britain/pull/160)is a one line update so that powerline tunnels can be built underwater. If I remember correctly, this is restoring the situation before PJMack's brilliant tunnels patch. This is helpful because the game sometimes builds power stations on islands out of reach of powerline bridges, as in this current example:
(https://i.imgur.com/nCzrxF6.png)
In this case, a causeway could be built, but that's unrealistic and I don't think there's anything preventing power stations being built on islands surrounded by deep water. In real life, powerline tunnels have been built across the English Channel, the North Channel, and the North Sea, which are the deepest seas modeled in this pakset.
It also adds more data into the BR info files.
Nice. (I've been playing mostly in the early 19th century recently, so hadn't encountered this issue.)
Actually, in real life, powerline tunnel is a bit of a misnomer, as it's usually just a cable dropped onto the sea floor, optionally buried under a metre of sand. There's no depth limit.