That 3rd set exists and functions fine in... Waterloo/Cedar Falls, IA, of all places.

When they took down the passenger rail way back in the day, they replaced the network with hiking/biking trails.

And in the winter, you have a cross-city skiing network.

Granted, there's plenty of space around those corn fields, but something tells me that if Cedarloo can pull it off, the richer cities can, too.