There used to be an MG line from Hasimara to Dalsingpara that was abandoned sometime in the 1950s after the construction of the Assam Rail link. (Dalsingpara is roughly at the mid point between Hasimara and Jaygaon). It was abandoned as there was no economic potential for the short branch line.
As far as a railway line to Bhutan is concerned - Bhutan is a hilly country requiring massive amounts of money for the construction of a railway. Economically, it doesn't make sense.