More useful in replacing cars in many area's, as using existing infrastructure, unlike trains.

Where I live, in Seattle, the bus system has been wildly popular over the past several years and the transit system has had high ridership growth (in total rides and as % of commutes) in North America while most of the peer agencies have seen total declines. The problem is that they stopped scaling:

- For a while Seattle has been unable to hire enough drivers to run the schedules, because there is simply more demand than supply of bus drivers in the entire US

- Seattle has also run out of space to store buses, and for various reasons people do not like having a bus depot located in their neighborhood (parking lots are unsightly, the bus depot generates traffic and lots of diesel buses are unpleasant; electric is not mature enough for widespread fleet replacement and it would be a massive capital cost)

- the amount of buses being run is now so high that buses get stuck in bus congestion, even with a whole four lane road dedicated only to buses in the downtown area, and several more bus lanes on parallel roads

So now we are hurriedly spending $50B to replace buses with rail, because

- trains carry more people in one vehicle and with one driver

- trains have full or mostly full separation from traffic, which means higher average speed and thus better fleet utilization and running less vehicles for the same frequency