Anki OVERDRIVE rails are designed to be pushed onto the sides of the track using clips that are built into the rails. If you find that the rails are falling off the track easily or if your cars are knocking them off the track, then most likely reason is because they are not installed properly.
Each rail has 3 to 4 clips that need to be fully inserted into the track. This will require a little bit of force to correctly clip them on to the tracks.
You can easily tell if a clip is inserted fully or not by turning the track piece over and checking the clips.
See illustrations below on how to check if the clips are inserted fully or not.