Although you'll encounter some hills, they aren't steep and the footing overall is smooth and fast. Enjoy some great altitude training with views along the way!
The Switzerland Trail follows the remains of a narrow gauge railroad that operated from the 1890's until 1919. The railroad once split at the town of Sunset (near the middle of the trail), where a spur line ran down Fourmile Canyon to the town of Boulder. It first served miners from Ward, Caribou, Nederland, and much of the region immediately around the trail. As mining collapsed, the trains focused on bringing tourists up for the great views and meadows (the "Switzerland" comes from marketing these day trips to tourists!) But cars become more widespread, mining dwindled, and expenses were high, so it was abandoned in 1919.
Because it was once a rail line, the grades are moderate. Some parts are a bit rocky, but much is smooth and fast. Vehicles are allowed on most sections, but traffic is very low. The scenery is sometimes stunning, with views of high peaks in all directions, aspen groves, and deep conifer forests.
Although this trail is great in either direction, many people just do sections of it for a shorter run. The section from Highway 72 near Nederland to the Sugarloaf Mountain Trailhead is the easiest part, with a nice downhill at first followed by nearly level grade near the end, all on a very smooth surface. There is also some nice scenery to the south about halfway through this section. The section from Sugarloaf Mountain
to Sunset is steep, rockier, and rougher than the other section, although you'll be rewarded with excellent views to the north.
From Sunset to Gold Hill Road, you'll face a long climb (over 1,000') with remarkable views and interesting history. The railroad built a hotel and park called Mt. Alto park near the top of the climb to entertain tourists long ago.
The last section from Gold Hill Road to Lefthand Canyon winds through forests and eventually becomes singletrack. It's basically flat, which is remarkable given the mountainous surroundings.