From Delhi, fly to Dehradun 5 hrs. From there take a taxi or bus for Uttarkashi which takes about 6 hours.
The closest train station is Dehradun. Take a train from Delhi to Dehradun. Taxis and buses are available from Dehradun. It takes about 6 hours to reach Uttarkashi. You can also reach Haridwar from Delhi and take a bus/taxi from there. Shared taxis are also available. The journey is very scenic, the Tehri Dam reservoir and the turquoise Bhagirathi river look surreal in the backdrop of the mountains.