If you think that building rail viaducts and rail grade separations are carbon expensive, wait till you learn about the carbon used for roadway viaducts and roadway grade separations.

You can argue that rails usually run on much more consistent of a grade than roadways but that's only to decrease the amount of acceleration that's required by train cars and to increase top speeds. Freight rail is a special case because freight companies try to avoid as much wear-and-tear on their train cars as feasible within their delivery windows, unlike passenger rail which necessarily cares more about speed and less about wear-and-tear.

Vastly fewer, per passenger mile, then passenger cars, comparable to busses, less than scheduled airline flights.