Transcontinental railway over several continents
Very ambitious project indeed. Tunnels are delicate enough to do right and railway over or under water is of the same order of magnitude. It has been done before - on the same continent.
Building such a project over multiple continents means taking continental drift into account. Continental drift is apparently an inch a year. A railway has a precision of ±0·5mm, which is about 200 times smaller than the drift.
That means that somebody is going to have to come up with a way to make what is a fixed structure float over the places where the continents drift - in opposite directions. And that solution is going to have to allow for high-speed travel and decades of use.
If they do pull it off, it will be a major engineering feat that will eclipse everything that has been built up to now.