content: Buses are some of the best vehicles for alternative power systems.  They travel regular planned routes, which means you can easily add specific charging/fueling stations and work them into the routes.  They're also high usage, which means you're getting more benefit out of changing them than you would a car that gets used an hour a day.  And if there's a way to invest high upfront cost in large long-term savings, it's easier for a metro system/company than an average person.