OK, but network rail don't have any in-house building skills. Actually they don't even do much truly in-house maintenance/fix work - it's mostly contracted out.

Network rail are not building east-west rail, let alone HS2.

True, but that is why I am sceptical about benefits of re-nationalisation. The skills are not going to come back.

The default assumption these days is that everything will be contracted out. It needs a cultural change and a lot of investment to change things.