Here is something I found on hybrid designs, it contains reference to hydraulic (they actually mean pneumatic because it uses nitrogen gas) regenerative braking system for cars and trucks like UPS. I would imagine that it could be used for buses as well.

http://www.cnn.com/2007/TECH/03/09/cars.100mpg.popsci/index.html

The EPA has built a modified hybrid that uses a hydraulic system, not a battery, to store braking energy. When you press the brakes, the wheels drive a pump that compresses nitrogen gas, which is inexpensive and inert. When you accelerate again, that compressed gas runs the pump in reverse to help power the vehicle.

Seems pretty sensible - CNG is the cleanest readily available fuel - it should be possible to fit tanks to the roofs of the buses ( unlike cars ), and plug in ( or any hybrid ) should work well for stop start driving - like a bus run.
Interesting to think how you might add CNG to double decker buses - where you do NOT have room on top - could you stick a "bump" at the rear somewhere and store it there.
In fact, surely all urban buses should be replaced (over time) by hybrids, even mild hybrids, for efficiency and pollution reduction.

And that was the beginning of stupid comments.