Lonely Mountains: Downhill

Each track has a Night Ride that is unlocked by completing the track's "Free Rider" challenge, which in turn unlocks when all the Expert challenges are completed.

EDIT -- It appears the Free Rider challenges are now available by default, so the Expert challenges are no longer required for this achievement.

The final Expert challenge is a combination of beating both a set time and a crash limit, and, while they are not easy, they are very doable--certainly on the early tracks--with a little practice. Fortunately, the first two tracks in the game are not too challenging and only require the Pacebreaker bike, which will most likely be the first bike you unlock.

The Free Rider challenge then requires you to beat the track without crashing, but honestly this is probably easier than the Expert challenge you just did, since you can stay on the trail and go as slowly and as carefully as you want. Finish this, and you unlock the track's night ride and the achievement.