A particular problem with projects that take decades (HS2, Nuclear subs etc) is that the original rationale (whether good or bad) and on-going plans are almost certain to be called in question and modified. Newly available technology, changes in available funding, changed priorities, political interventions, escalating costs, perceived public support for the project etc will throw projects off course.
Government's only last 5 years (or less) unless reelected and even if the same party continues, the transport minister of gets moved on and the new one wants to make his mark!
Perhaps making something deliverable in 5 years even if it means temporary connections to the existing infrastructure might be the way to go?