I don't know about the US, but in the UK one reason to avoid building on old railway lines is that they might technically still be railway infrastructure. For

I don't know about the US, but in the UK one reason to avoid building on old railway lines is that they might technically still be railway infrastructure. For example, the newly-rebuilt Borders Railway, south from Edinburgh, was only able to be built because in a limited sense, it had never stopped being a railway so many of the required permissions were already in place.