Started trip from Kolkata Sealdah station.
From here a train runs daily to Hasimara which is the 3rd last station towards the north east of West Bengal from where it is easier to follow up further towards the border road entry point of Bhutan namely Jaigaon - Phuentsholing.
The booking of the train has to be done before hand which can be done directly from the Indian Railway Portal www.irctc.co.in or from any other train booking sites as well. I used Paytm to book the trains.
Train running: Kanchan Kanya 13149
Usual timing: Daily 8:30 PM
Lowest fair: Sleeper Class Rs 370 approx
Highest fair: 2 AC Rs 3000 approx
Travel Time: 14 hrs +