>>1097002>carsYes. But this is already being done, the concept is called ring road. While it's not always underground, especially when the ring is far from the city centre, there are notable examples or rings going in big part underground. For example A86 around Paris, France contains the world's longest urban motorway tunnel. They also rarely contain at-grade crossings.
It should also be kept in mind, that such constructions are extremely expensive. Personally I would get rid of cars out of the city completely.
>trainsIf you mean trains, in main cities they already are routed in such a way, that they are barely visible. Most of the time they are fully grade separated, so they either run underground or over bridges to leave the city centre.
If you mean trams, then hell no. They are one of the best methods of transport overground in a city. The stops are close to each other in the centre, making it easy to reach one from any place; the frequency and possibility of having multiple lines sharing part of the route make the waiting times really short; they can reach far into middle rise and suburb areas making urban sprawl and commuting feasible; they almost always pass by some major hub(s), extending the coverage of heavier transport, like commuter rail and metro.
Role of commuter rail is to connect far suburb areas and surrounding cities with the core metropolis, in case of metro it's often to connect suburbs and provide fast travel between points distanced far apart.
