"Just get it done". I seem to recall that phrase in a recent political context. It was a fine sentiment, until the details of getting it done started to dawn.
Isn't that similar here? It's not at all simple. HS2 is just the start.
I have a Sheffield based orientation on this. Just to bring HS2 into Sheffield on a spur needs at least the tripling of the lines down the Sheaf valley from Dore. Restoring 4 tracks is no longer realistically viable due to encroachments on the old trackbed, although loops could help. That part needs doing for any electrification scheme. Just to get from Chesterfield may require Dronfield station to be moved south. There are a significant number of road overbridges that are likely to need rebuilding and Bradway Tunnel probably needs attention too.
Sheffield station badly needs remodelling, but for what and how? The current passenger bridge across the station is relatively new but needs rebuilding to offer a gated half for rail passengers and a second separate ungated path for non-rail users. The now disconnected south facing spur at the north end needs reconnecting to allow active use to stand terminating services awaiting a return south or west. Bay Platform 2C needs lengthening from 2 car at the south end. How capacity to the north can be increased from the station into and along the Don Valley is challenging.
I could go on with more details, but add one more. Rivers. The line from the south criss-crosses the River Sheaf on numerous bridges all the way down from Dore to Sheffield station. That is on a giant complex raft of stone and brick arches built over the site of old ponds, beside the neighbouring bus station in Pond Street and the swimming complex at Ponds Forge. The Sheaf continues beneath more arches below Park Square/Ponds Forge roundabout to the River Don. Here's a picture of the trap door between Platforms 2C and 5A from below.
The provision for storm water is massive, yet detritus is suspended quite high up on the stone and brickwork along the way, this picture taken in one of the three adjacent parallel tunnels after a long period of dry weather. There's a constant rumble of trains overhead. All part of the hidden infrastructure liabilities and costs of our railways.