UPDATE (Aug. 24th, 2012): Apparently the servers are still up for the time being, so this achievement IS currently obtainable, not sure about the rest of the online achievements though, so while this is good for those who still need to finish online (which I suggest you do ASAP), I would not recommend anyone who cares about completion or completion percentage to start this game at this point without further research.
UPDATE (Apr. 14th, 2012): Sorry folks, as of April 14, 2012 this achievement is no longer obtainable due to server shutdowns, sorry to all those who were left hanging.
This achievement is awarded for winning 50 Races online, either Ranked or Player, and Private player matches do count toward these. They must be Races, Road Rage races no longer count after the update. Since this game no longer has very much of an online community, I suggest you boost this achievement, you can find a
Gaming Sessions or set one up yourself.
The Set Up I used:
Main Menu > Xbox Live > Create Match
Ranked: No
Game Mode: Race
Vehicle Top Speed: 209 MPH
Laps: 1
Rounds: 10
Security: Private
Track 1-10: Motor City Short F
Each round counts as a separate race, I then would invite my boosting partner into the game and we would alternate 5 and 5 winning the races. Rinse and repeat 10 times and you will both have the achievement.