I've repeatedly said that if you need a high capacity vehicle with just one driver, over a sustained period of time, you probably have a good reason to build rail. Rail's ability to expand vehicle size through multiple cars, without adding staff, is one of the ways it works well on the bottom line.
But now we're seeing some really, really long buses:
Are there limits to the length of the bus that do not apply to rail? Yes, though it's easy to imagine cases where they wouldn't apply, such as median busways where the bus never has to weave to access a stop. In ordinary operations with curb stops, I suspect that we won't see buses grow much longer, and there is even interest in replacing the articulated (single-bend) bus with double-deckers, in the London manner, to use urban curb space more efficiently.