There is a difference actually between NJP shatabdi and Patna VB. Darjeeling bound passengers hardly use NJP shatabdi actually. NJP shatabdi mainly acts as a Intercity service between Siliguri, Malda and their state capital Kolkata. Kolkata being the state capital, a lot of passenger movement (not tourist) takes place between Siliguri and Kolkata. In case of Patna VB, that factor is missing actually since both Siliguri and Patna are in different state. It will surely get Parna bound passengers from Katihar and Kishanganj but the passengers from Siliguri will be mainly tourists visiting Darjeeling.