It depends on the country where the ticket got issued. Expedia is using different reservation systems.
Germany: used to be Worldspan, is now Amadeus
United States: used to be Worldspan, is now Sabre
Some other countries still run on Worldspan...
Sabre:
www.virtuallythere.com
Worldspan:
www.mytripandmore.com
Galileo (not used by Expedia to the best of my knowledge):
www.viewtrip.com