I was more thinking of separated bus lanes. The idea being that if you can create a separated railroad in a space, you can create a separated bus lane in that same space for significantly less cost.
Buses are great for roads where there isn't much traffic. However if traffic is bad you have enough demand that a train is better investment.
I am strongly coming to suspect that a lot of the talk about buses being all you need is fueled more by a sort of NIMBYism than by a fully-informed position. Buses are great and good transit systems need more of them than they do trains, but an ideal urban transit system has both.