it depends upon the time you travel, if its now, as per newspaper reports, its raining heavily in AP and train timings may be affected. during winter months in delhi, schedules go haywire due to fog.
even if you are not able to take the train, there are plenty of buses every 15/20 mts to Dehradun from ISBT, Kashmere Gate. You can travel to Kashmere Gate by Metro whch is available from NDLS (near Car Parking in Ajmeri Gate Side/ after Platform 16) and take any bus towards Dehradun.