They lacked the internal combustion engine. It’s a fairly significant upgrade to the initial idea of the highway.
Railroads for individual transport are like circuit switched networks, while cars on roads are like the Internet, with cars as the packets. There are lots of good reasons why the concept of packet switched networks won out over circuit switched networks, both in electronics and in transportation.
I like to make the point that people driving themselves around in (or on) their own personal vehicles is a form of mass transit, and a pretty efficient one, too. Until you get into congested urban environments.
The internal combustion powered automobile debuted in 1885, so I guess it’s a 19th Century solution to 21st Century problems too.