what would be the best way to phyisically connect a series of computerised bus stops to a central computer in a city-wide network??

Smells like homework to me.

haha its not homework as such... more of a personal project. I've decided as its going to be over a large area a Wireless WAN would be the best option but I’m not sure of the benefits this would have over a wire based network (apart from the obvious fact that its wireless :P) also what sort of methods (apart from basic firewalls etc) could be used to keep the network data secure?

even the most basic of help would be appreciated.

Firewalls etc are just the start, you don't want anybody to be able to DOS the terminals just by sending them lots of junk packets or whatever. The less you expose to the rest of the world, the better. For security, it might even be better to use some obscure frequency or protocol which would-be intruders might not even detect simply by bringing a laptop and run wireshark for a while.

For securing the actual communications, I guess regular VPN solutions should work.

You might get more useful feedback from some embedded developers forum or some such. If you are going to be using e.g. QNX or something then those guys probably have lots more field experience of similar deployments.