If you have to travel from Nainital/Kathgodam to Kolkata , then kindly don't take this train directly even if the seats are available , irrespective of class.
Instead book this train from KGM to LJN (For this journey you can opt any class), and from Lucknow to Kolkata , book Whichever train you are getting confirmed seat:
1. Upasana/Kumbh Express. (Class doesn't matters as SL class is also very safe and good to travel).
2....
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Doon Express. (Book only AC. Never go for SL class)
3. Punjab Mail. (Book AC class only. Non AC class are pathetically crowded).
If you opt for this break journey , the benefits are:
1. Your precious nearly 7-8 hrs of extra journey will be saved.
2. You will be saved by the pathetic crowd comprising of unreserved passengers and WL holders in Bagh express between LJN and CPR.
Your Journey should be:
Board the Bagh Exp train from KGM , go to your seat , take a sound sleep , and deboard the train at LJN.
Freshen up yourself at Waiting room in LJN/LKO , do your breakfast, and then go to LKO.
Board your Train (Any one of the above 3) and reach Howrah by next day early morning.
PS: I have guided for Lucknow as a change over point because the buffer time here is perfect with perfect amount of amenities. Whereas if you take MB or BE as your change over point , then your night's sleep will be wasted. This change-over "life saving" trip has been done numerous times by me and by my dad also (when he was there in Kasauni).