Nearest rail head is New Jalpaiguri and airport is Bagdogra,around 4 hr by road from Gangtok.No daily train to Chennai,weekly trains on Friday,Monday and Wednesday but will take around 40-45 hr.One option is to break your journey at Kolkata,many daily trains/flight to Kolkata and then again daily trains to Chennai.You can check train details https://m.etrain.info/in-TBS here. If planning to use train then you need to book in advance.
No direct flight to Chennai but can take flight via different city like Kolkata.
Thanks so much. We plan to do a few stops along the way
Thanks would Like to go to Bhubabneswar, thought maybe in September as we will be on our way back towards Kolkw
Great to hear someone traveling all the way from Sikkim to Chennai! That includes a lot of stops. You can find many hostels along the way, and you may book them through Indian hostel booking website AAO Hostels or international websites like hostelworld. You can also download mobile app through Google Playstore.
Hope you enjoy your journey! :)