Okay, maybe not bus manufacturing, but the point here is broader than that.
I've spent about a quarter century in the transit business, 14 years of that as a bus driver. I've driven vintage GMCs, Mercedes, and Carpenters, and new-at-the-time Blue Birds (they used to make transit buses) and Gilligs. That phase, driving, ended in 1997, but since then I've mostly gotten around by bus and bring that experience as a driver to the table in evaluating the buses I've been on as a rider.
To distill that experience into a single observation, it seems to me that bus manufacturers must have a difficult time hanging onto good engineers. (If you're an engineer, working for a bus manufacturer, how many resumes have you sent out in the last year?)
Bus manufacturers are primarily two things: they actually build the coaches or have them built to spec, possibly starting with a purchased frame, and they act as systems integrators, incorporating major components like engines and transmissions as well as minor components like signage and security systems, delivering it all as a package.
Each of these components may have troubles of its own, but in addition to that there are typically problems with the integration of components, and during the break-in period a good deal of debugging must be done before the buses are really ready for service.
In addition to the above, the underlying technology is on the cusp of change, with electric drive having become a practical, affordable alternative to mechanical drive, and rapid progress in battery and supercapacitor technology, as well as sensors, controllers, and electronics of all types. The list of potential improvements is practically endless, but the pace at which they are actually finding their way into each year's new models is embarrassingly slow.
This is a situation ripe for an Apple-style intervention, much as they intervened in the cellular industry with the iPhone, setting the bar higher for the industry as a whole.
It might be a bigger deviation from their core business than Apple would care to make, and there might be another company better positioned to step in, but that notion of intervention still applies.
For Apple such a move would have a profound effect on their brand, assuming they were to use "Apple" in this context. (Note that they don't with FileMaker, which is much closer to their core business.) Initially it would be incongruous, but over time people would come to think of Apple as an approach rather than as a product range, so it might actually be beneficial. Moreover, it would be bold, and people admire boldness.
With respect to Apple, the real point here is that one shouldn't count out the possibility of them doing something as surprising as getting into bus manufacturing, because it isn't a safe assumption that they won't, and the sluggards in whatever industry they do move into had better sharpen up their game if they don't want to be left in the dust.