i designed a system exactly like this for my last company.
First, lets discuss the central side.
The central side will include a server that has a connection to the GPRS network. The server will perform the calculations determining the next stops and time until arrival. The server will transmit this info onto the GPRS network, being received by the Vehicle Display and Wayside display (bus stop)
The vehicle display should make a wireless connection to the GPRS network and a wired connection to a GPS receiver. The vehicle should transmit its location data every 10-60 seconds (depending on the system load) to the central side. The central side will process the data, and transmit display information back to the vehicle.
The wayside display should also make a connection to the GPRS network, but no need for a GPS interface, as the wayside will be at a known fixed location. The display need not transmit any information maybe except a status message or error messages. The display will receive information from the central side based on the calculations made from the vehicle info.
Any questions?