What do you mean by "central booking system"? As far as I know (working on a rental car contract for a couple years), there are just various travel industry standards for bookings and whatnot.
GDS.
A Global Distribution System is a worldwide reservation system that acts as a conduit between travel bookers and suppliers, such as hotels, other accommodation providers and other travel related services.
I mean there is only one pipeline, to which are contributing many actors, like big agencies (booking, expedia), car rental, airlines, etc. Together, they act as one.
It started with airlines, but eventually included everything from hotels to rental cars.
The space is more or less dominated by a few big players. Sabre. Amadeus. Travelport.
GDS is the backbone of all of these booking platforms as far as I'm aware.