Roads are just asphalt. Train tracks need all kinds of supporting equipment installed for safety. If you want to electrify the track I'd guess that just installing that is more expensive than building a road. I'd be happy to be proven wrong though, do you have any sources about the average cost per kilometer of roads vs tracks?

No they are not, there’s all the signage, barriers, and drainage. Roads wear out very quickly, they are the fastest degrading forum of infrastructure.