Just a quick note: The Si trail, like other popular winter conditioning hikes, e.g., Tiger and Mailbox, becomes slicker than deer guts on a doorknob as happy hikers compact even light snow into ice as trecherous as any late season glacier. Recommend at least ski poles and, much better yet, insteps or even real crampons, and knowledge of their use. Yeah, it looks silly, but ice is ice and even minor injuries can mean major down time.
Wishing you upright hiking,
Jonathan