The rails are the easy part. There's also signalling and monitoring, power gen/distribution, train stabling and maintenance, and all kinds of other infrastructure.
You can't build it piecemeal without incurring huge extra costs.
The insane part with HS2 is the relatively small economic benefit, and the fact that the price is going to spiral out of control even more than it has already.
The UK badly needs an integrated transport policy, making connections between all modes - trains, busses, airports, ferries, roads, and so on. Currently all the modes compete with each other. And that's a huge problem.
Instead we get a project that might have been progressive in the 19th century but makes no economic sense at all in the 21st.
For the same price as HS2 the UK could have hugely improved broadband everywhere, improved rail - especially electrification - and seed funding for new businesses everywhere, especially the North.