Most Delhi-based travellers fly directly into Srinagar. If you'd rather travel by train, the Jammu Rajdhani/other trains connect Delhi to Jammu Tawi, followed by a scenic but long road journey (roughly 8-9 hours) to Srinagar via the Jammu-Srinagar highway.
Suggested Itinerary Length
Given typical leave schedules, 5-7 days total (including travel) is the most common trip length for Delhi travellers.