If it turns out that we're still using underground trains in 100+ years time then that would be interesting in itself. Hyperloop [1] maybe?
People in 1915 were I am sure imagining a future of personal flying machines which they were confident would be in operation by 2015.
Here we are, still cramming into the same tubes that people were travelling in (albeit probably less crammed) over 150 years ago.
Flying cars were a means to an end; we got a similar end, we just found a different means.
[0]: https://medium.com/@timrpeterson/140-characters-is-as-good-a...