I've seen it estimated elsewhere that a km of heavy rail track is slightly cheaper to build than a single freeway lane. To what degree all the surrounding infrastructure, maintenance and externalities change that equation I'm not sure, but a single track is surely capable of moving a lot more passengers per hour than one freeway lane. Of course there may be relatively few situations it actually does, but if your goal is to get as many people from A to B as quickly and cheaply as possible, rail is surely cheaper.