Like everything on trains, it's complicated.
You could design trains, where all the wheels are driven. I expect lighter rail solutions could be doing things like that.
One of the trains where I was born, were self-propelled cars run on diesel. The longest of those trains, were three self-propelled cars connected to one another. An "engineer" was located in a short cab area, and was in control of propulsion settings. That train used to do 60 MPH when outside of the yard limit, and did as well on the tracks it got to use, as any national rail trains did.
The number of cars the provider would put on, depended on how many passengers were expected. At it's height, they would put on three cars, and it took something like six hours to reach the end of the line. The train would then return the way it came and arrive back in the originating city.
Doing it that way, means you don't have a loco that weighs 400,000 pounds. The wheels are still steel.
You can see it even has its own air horns on the car. And the train does sound horns at level crossings. But the service life of these is up, and they're no longer running and no similar replacement was designed. But while they existed, these were excellent. You can see the car also has an oscillator-light on the front, which rotates a bit, presumably to attract the attention of careless pedestrians.
Paul