I already said it. Highway expansions around me happen with couple of detours or some close lanes. Railway expansions require complete cancellation of services. I remember when Poland was renovating their trains they signed a deal for new Pendolino trains that didn't have the ability to lean on tight turns. This limits their top speed to 160km/h if I remember correctly. Increasing speed would require changing all the trains. In Spain some trains have to be build for specific lines because tunnels there have different size. There are many incompatible system which mean that choosing one or another locks you in for decades (I think BART has this issue). You really don't have those issues with cars.