nearest station for Char Dham Yatra: YNRK
If you want heli service you can go frm here easily to Jolly grant airport which is around 20 km and from Dehradun it is 25-30km. For DDN you need to detour, which is not worth, while returningyou can come to DDN nd visit Mussoorie..
For roadtrip go for YNRK/HW, taxi are available at both places, roads are also nice these days(till Srinagar garhwal I hav idea)