There's atleast one bus plying daily from Patna to Guwahati (Patna_Guwahati) and also in the reverse direction from Guwahati to Patna (#Guwahati_Patna) via Siliguri, Darbhanga. 

No further details are available. 
. 
. 
. 
* Indian Roadie 
* Northeast Roadie 
* Bengal Roadie 
* Bihar Roadie 
* Indian Roadie Bus Route