That is unlikely to happen. The railline would come till Rangpo i guess which is 1.5 hrs below Gangtok. So the main city would be spared. Another thing is the Sikkhim govt has this great policy of providing opportunities for localities. To put it in simple terms, you cannot own a property in Sikkhim or get a govt job if you are not from the state. The locals too are smart. They respect the outsider as long as the outsider respects you , your region, your culture etc unlike in other cities where migrants are allowed to stay on their own terms even if they disrespect the local sensitivities.