The HS2 proposal was for an underground station beneath the current Old Oak Common depot and sidings, just north of the Great Western main line, with a passenger interchange there with Great Western and Crossrail. This map from Wikipedia shows where the station would be (click on it for a large image in a new window/tab).As I recall, there were vague mentions of interchange links to other lines later, but exactly how this would be achieved is not clear. As you say, Chris, the various lines are well separated, and the resulting mega-interchange might not be very convenient without expensive re-routing of lines that would be difficult to cost-justify.
Richard Jennings ● 5856d