Crossing the railroad tracks is technically illegal. Boulder has been planning an underpass for some time but no movement as of yet. Cross at your own risk.
The Cottonwood Trail is a wide, crushed gravel doubletrack. It parallels Whiterock Ditch for most of its length and gets some nice shade from plentiful cottonwoods and willows.
Cottonwood is best used as a connector to get to other trails/paths.
Continuing south past Independence, the trail becomes a paved path, which can be used to get over towards Valmont Bike Park. Near the halfway point, a short hike-a-bike over the railroad tracks on a well-worn "trail" will connect you up to Four Mile Creek path to Pleasant View Soccer Fields and on up to 26th Street.