The gate by the Observatory is only open weekends 10 am - 5pm.
A singletrack downhill that connects with Newcombs Pass, Gabrielino Trail, and other trails. It's great for a DH shuttle.
Enter through the gate at the top of Mt. Wilson (this gate is only open on weekends) and continue east towards the observatory. You'll see the Rim Trail on the left.
From the top, the trail points east and stays fairly flat with for a while and then drops via switchbacks into the canyon and ends at Newcombs Pass. There, you can catch the Gabrielino Trail to Chantry in Sierra Madre. The trail can be very thin in places and can be covered with leaves or snow, so stay alert.