You don’t want to get in the hardware business here and in most countries you are probably not even allowed to deploy your own raspberry-pi based solution in a public bus, because it doesn’t fulfill the necessary security requirements and certifications.

That is why you have specialized sub-contractors who provide you with tested, certified, ruggedized hardware which you can install into the vehicles. It will also have connections for most proprietary ports you find in those environments. Of course it’ll cost you.