The plural form of bus is buses. To be fair, a few dictionaries do list busses as an alternative plural form of bus. But it appears so rarely that most people would view it as a spelling error.
The buses start running early in the morning.
The busses transport passengers to work.
Thirty-two buses in the Texarkana School District’s fleet were outfitted last week with the Zonar tracking system, which tells parents when buses pick up and drop off their children.
Retrofitting existing buses with new seats and belts would be “extremely expensive,” Fahrendholz said, estimating such a retrofit at $25,000 per bus.
Trams, after all, are basically less flexible buses : they take up space on roads, they’re relatively low capacity, and you can’t make them go where you want.