By RoadThere are three land border crossings located along the southern border that is linked to India only. Phuentsholing, Gelephu and Samdrup Jongkhar. Road permits are required if you wish to get in by road. These permits can, however, be processed along with your visa.
By TrainThere are no railways in Bhutan. Travellers, therefore, has to reach the nearest railway station and then take local transport or a bus or cab to reach Bhutan. The most convenient, and also the best, option out of all is entering Bhutan from Siliguri, New Alipurduar, Hasimara or New Jalpaiguri in India. Hasimara is situated at a distance of 15 km from Jaigaon, the border town on the Indian side of the border (near Phuentsholing). So, you can reach Hasimara by train, and then take a cab to Phuentsholing, where you will have to take a permit from the check-post to enter places like Paro and Thimpu in Bhutan. New Alipurduar railway station (North Bengal) is situated at a distance of 60 km from Jaigaon. You can take a bus ride or a taxi ride from New Alipurduar to Jaigaon.
By AirThe Paro International Airport is the only airport in Bhutan. It is located in the south-west of the country and caters only to Druk Air with flights connecting Bangkok, Delhi, Kolkata, Bodhgaya, Bagdogra, Guwahati, Kathmandu and Dhaka to Bhutan. The fares are a little steep. However, it is also widely considered to be one of the toughest airports in the world to land. Bhutan has three other domestic airports. In the south-central region is the Gelephu Airport, Yonphulla Airport is located in the eastern side of Bhutan, and the Bathpalangthag Airport is in the central part of Bhutan. However, flight operations are now permanently closed in Yonphulla and Bathpalathag airports.