I imagine it would work just the same as wifi on buses currently. There will be a mmWave access point inside the bus which will use something else (maybe a cell

I imagine it would work just the same as wifi on buses currently. There will be a mmWave access point inside the bus which will use something else (maybe a cell backhaul?) for it's connection. As someone mentioned in another comment, the best use case for mmWave is places with very high density such as concerts, sports stadiums, etc where current cell networks and wifi seriously struggle. You don't have channel congestion if the waves can't make it through the walls in the venue.