Hi Anamika.......!!!
Everyone has guided you correctly...
There's no Deluxe bus service available for Dehradun side....
Trains go via Saharanpur, but you will have to board one from Ambala Canttonment Railway Station.....
And never ever prefer a local ordinary bus.... Its not at all good to go via an ordinary bus, especially when you are girl....
The best way out is, take an A/C Taxi for Dehradun...
It will Cost you something around 2000-3000 bucks, but thats the safest and the most comfortable mode to communicate between the two places....
I think you are new in this place hence can't even ask you to drive down, b'coz that is the second best alternative.....
And there are no Taxes for anything as such...
Anyways hope you Enjoy your stay both at Chandigarh and Dehradun....
Bye and Take Care......!!!