updated 9/20/2021
A new bus service called Trailhead Direct can take you directly from downtown Seattle and the Capitol Hill light rail station to the Mt. Si and Teneriffe trailheads. The Trailhead Direct buses run on weekends and holidays in summer. This bus service must be considered experimental at this point. However, if it is successful, Trailhead Direct could be the dawn of a new era for the bus hiker. For more information, see:
https://trailheaddirect.wordpress.com/
Note that there is another Trailhead Direct bus that starts at the Mt. Baker light rail station and heads for Issaquah-area trailheads - don't confuse this one with the Mt. Si bus.
On weekdays or any time other than summer, you will have to reach Mt. Si using other buses - see the chapter in this guide:
https://sites.google.com/site/seattlemetrobushiking/main-page/hikes-in-east-king-county-1/mt-si
The first trailhead that the Trailhead Direct bus accesses is the one for the Little Mount Si, a fine hike in its own right. Then the bus arrives at the main Mt. Si Trailhead. From here, a well-constructed and graded trail climbs 3500 feet in 4 miles to reach a spectacular viewpoint atop Mt. Si. The true summit of the peak is reached by a dangerous scramble up the rocky "haystack", but the view from the base of the haystack is perfectly fine. You won't be alone up there; this is an extremely popular hike.
After visiting the Mt. Si trailhead, the Trailhead Direct bus arrives at a third, newly constructed trailhead for the Mt. Teneriffe Trail. Two popular hiking destinations can be reached from here - Teneriffe Falls (often called Kamikaze Falls) and Mt. Teneriffe.
Teneriffe Falls is a moderate hike of 2.6 miles (one way) and 1600 feet of elevation gain. On the way, you will enjoy expansive views out over the North Bend valley. The falls themselves are spectacular in spring during the snowmelt season, but nearly dry up later in summer.
Mt. Teneriffe is an extremely strenuous hike that gains 4000 feet in 7 miles, mostly on a decommissioned logging road. The view is grand from the top. Allow plenty of time for this hike!
This trail is much less popular than Mt. Si and Teneriffe Falls trails, nice if you don't like crowds.
Several side trails connect the Mt. Si Trail to the Mt. Teneriffe Trail, making various loop hikes possible. You can get off the bus at one trailhead, and catch a bus at the other. A good map can help you sort out the possibilities-the Green Trails Mt. Si map is a good choice. These interconnecting trails are less-used and offer quite a bit of solitude compared to the Mt. Si trail.