You can certainly do it with an RPi - although you might have to keep rebooting it every hour or so. :)

GPS and 3G/4G are trivial add-ons. There are stacks for mobile data.

RFID doesn't have the reliable range, so that won't work as a solution.

It's not really a hard problem for a competent small engineering team with a mix of embedded hardware and software skills. It's not quite a trivial problem, because there are some interesting edge cases, mostly when a bus is diverted. But there is absolutely zero rocket science required.

You could hack something unreliable together in a weekend, but it would take up to six months to get all the bugs out, do the production engineering for a bullet-proof solution, standardise the installation process (if retrofitted), and so on.

Double that if management is poor. :)

Those last parts often get forgotten in software. Hacking something together that kind of works if you don't look at it with a visible frown is hobby coding, not engineering. Engineering is the boring work that happens after that, to build something with a quantifiable many-9s uptime that meets or exceeds a standard spec with comprehensive test cases.

Usually the announcement is not "we have stopped at Foo and Main". The announcement is "Next stop Foo and Main" while you're on the way to it, so you have time to sling your bag back over your shoulder, get your coat back on, and make your way towards the door. Depending on the density of stops along the route this can come anywhere from 1-7 blocks ahead of the stop.

Source: I don't drive,and ride the bus a lot.

You generally want to make the announcement before the bus stops so that people can be ready to get off when the bus stops. Also buses don't always stop at every station on it's route so have to deal with what happens when stop is skipped.

That sounds expensive.