There's an interesting gradation from buses to trains. I made a picture to show this: https://pics.dllu.net/file/dllu-sc/d089a0155a.png https://pics.dllu.net/fi

There's an interesting gradation from buses to trains. I made a picture to show this: https://pics.dllu.net/file/dllu-sc/d089a0155a.png https://pics.dllu.net/file/dllu-sc/d089a0155a.png Everything on the left of the dotted line can go without rails/guidance and everything on the right has to stay on rails. 1) regular buses 2) buses on specialized roads (busways) 3) guided buses which have extra wheels or electronic guidance to follow the busway without the driver steering: https://en.wikipedia.org/wiki/Guided_bus https://en.wikipedia.org/wiki/Guided_bus 4) bus/tram hybrids like the Bombardier Guided Light Transit, which usually operates with a guide rail but has the ability to go without any rails: https://en.wikipedia.org/wiki/Bombardier_Guided_Light_Transit https://en.wikipedia.org/wiki/Bombardier_Guided_Light_Transi... 5) rubber tyred light rail systems (e.g. automated guideway transit systems, often seen at airports) 6) rubber tyred heavy rail (e.g. Montreal Metro) 7) heavy rail The lines between buses and trains will only get more blurred in the future with autonomous bus platoons and the such.