Yea I think since roads are paved over more often depending on the city, streetcar lines are harder to find and not really as noticeable. When I was growing up, this intersection in Detroit had exposed streetcar lines. It's since been repaved many times but I'm not sure if the rails were ever removed. Probably not.
http://g.co/maps/nnmrr
There's still some exposed on the southwest side of the city though.
http://g.co/maps/ek9zs
And some better preserved ones in Philadelphia.
http://g.co/maps/7ssy5
As other posters mentioned, streetcar lines typically follow the flow of traffic whereas freight lines had their own right-of-way.