In an open track, top end would be the speed of sound at sea level, moderately slow. But that’s probably a little too fast for the current engineering.
There’s nothing wrong with 250 to 300 mph. If you have further to travel you might want to go higher in the atmosphere, or leave it altogether.
At ground level, the only way to go faster is in an evacuated tunnel. If we combine evacuated tunnels with electromagnetic propulsion, or even simple pneumatics, we could get appreciable speeds between cities. Think of the elevated railway.
The main thing to do with these slower modes is to have a way of sorting the people as they travel, so that each can get to a selected destination with a minimum number of stops or transfers. The same principles apply here as to a really massive high-rise, where local slow elevators take people to an interchange floor where they can move to faster routing for a period, and then get another local to their selected location.
Such elevator systems could integrate with surface transportation to get a traveler from one massive building in one city to another in a different city many miles away in just minutes.
Bob have not seen any plans but a line is in the works/plans for NC. Bet the military has some in use (opps did I say that) if one or more are in the works for citizens