I take a commuter bus for part of my commute. There’s some variation in the actual vehicles that run the route, but it’s usually a diesel bus, I think, and one of the “kneeling bus” types – the ones that can lower the front door of the bus so that passengers with mobility issues can board more easily.
I assume that’s accomplished by manipulating air pressure in pneumatic suspension or shock absorbers? The pneumatic sounds it makes suggest as much, although the accompanying blast of an air horn almost drowns them out (presumably to warn people the descending bus might pinch their foot against the curb?)
Almost every morning we pick up such a passenger, and when returning to the fully-upright position, the bus makes a series of these air-pumping sounds – it’s not a steady sound at all – usually three long wheezes as it rises. Then the bus turns off for a moment – the lights go out, the engine stops. Then the engine turns over and begins to rumble again, the lights come back on, and there’s one final very short movement upward, accompanied by an equally short toot of the air horn.
What’s going on there? Is the bus actually being turned off and restarted most of the way – but not all the way – through the process of returning to its original height? Why would that be? Is it by design, or is it a quirk of one particular bus?
It’s not an occasional thing; with these buses, it happens every time they “kneel.”