Very soon, travelling to Sikkim by train will be a possibility

A direct train from Sealdah to Rangpo in Sikkim might soon be a reality if things go as planned. The Indian Railways has already set the commissioning date for the said 44.9 km Sivok-Rangpo project for 2021, and work is in progress to meet the deadline. Once completed, this train journey will offer you breathtaking views with bridges and tunnels on the way.
As of now, the only feasible way to reach Sikkim is the road route, i.e, Sivok to Rangpo, whereas the new Pakyong Airport is operational only when the weather is favourable.

Referring to this development, an official from the Northeast Frontier Railway (NRF) said that traffic gets stuck on the Sivok-Rangpo route frequently because of landslides and other reasons. However, the planned train journey will be less than two hours, which will cross Riang, Teesta Bazaar, and Melli stations. Travellers can simply take this train ride, and hire a vehicle to take them to Gangtok or some other location. As per the reports, tracks will be laid up to Gangtok during the second phase of the project.
Several issues, such as land acquisition and environmental clearances, kept hindering the project since 2009. Reportedly, 85 per cent of the total train journey will be through tunnels, with a total of 14 tunnels. While the longest of these will be 5.27 km, the shortest will be of only 538 m. The route will pass through the forest divisions of Darjeeling, Kurseong, Kalimpong, and East Sikkim and also through the Mahananda Wildlife Sanctuary. The tunnels will be built to ensure there is no impact on the flora and fauna. Apart from the tunnels, there will be 19 bridges along the picturesque stretches.