Actually it turns out rail lines have fairly low capex compared to many other transport types. It makes sense once you think about it, but I was surprised too.
Many lines used to have double or quadruple or even electrification but are now single unpowered tracks, because the additional improvements counted towards property tax assessments. It's just not a level playing field.
ps: obligatory history article https://www.lowtechmagazine.com/2008/02/a-world-without.html
I guess no one is building one lane freeways, but I was still surprised by how much more expensive rail is.
(This is just the result of a basic Google search. It also says maintenance costs are "much less" for the railway.)
There are probably economy-of-scale issue too (we build a lot more roads), and different expectations of road vs. rail safety (a rail accident with a few injuries has national media coverage for days, upgrades costing millions may be rolled out to prevent a repeat; every day several people die on roads, the infrastructure is rarely improved).
sort of like a shared packet network compared to a centrally managed circuit switched network