कोशी का प्रवेश द्वार — मानसी जं० - Prabhat Sharan

Its an excellent idea to travel by trains to the maximum extent possible. However, please bear in mind that train reservations are difficult to get unless done quickly. In most Indian trains, reservations are allowed 60 days in advance and its possible that all available berths are filled up within minutes of bookings opening. So you should aim to get all your bookings done at 8 AM Indian Standard Time on the day the bookings open. Since you will be using a foreign debit card (not issued in India), you will have to book via Cleartrip or Makemytrip as the official website IRCTC.CO.IN does not accept foreign cards (as far as I know).
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13023/ HWH to BHP
1st AC is not available so you can choose 2nd AC class of travel.
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2. 13149/ BHP to SGUJ
1st AC is not available so you can choose 2nd AC class of travel.
Only 4 seats are available in this quota on this section so you must really book fast or you risk getting into Waitlisted situation which can be unnerving for a first time traveler.
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3. 15651/ NJP to JAT
1st AC is not available so you can choose 2nd AC class of travel.
This segment is an intermediate sector on this train i.e. its not end-to-end and hence has a very small number of seats. The chances of getting a confirmed berth if not booked at 8 AM are very low. If you don't succeed, you have the option of booking a reservation from the source station Guwahati (GHY) and selecting NJP as your boarding point. While this is possible on the IRCTC.CO.IN website, I do not know if a different boarding point can be selected on Cleartip or Makemytrip so you might want to check that.
If you do book from GHY and do not have your board point selected as NJP, then you will lose your seat entirely as it will be alloted to somebody else from GHY. Do not assume that you will be able to board from NJP without officially having NJP as your boarding point on your ticket.
You always have the option of traveling from GHY itself instead of NJP.
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4. 12414/JAT to AII
1st AC accomodation is available on this train. It only has 2 seats in General Quota it seems (there are more seats allocated to other quotas which you really shouldnt be worried about at this point). So you need to book these quick too (i.e. at 8 AM IST 60 days before the travel date)
If you fail to get 1AC, you can also avail of 2AC class of travel.
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5. 12988/AII to SDH
1st AC is not available so you can choose 2nd AC class of travel.
Please note that this train will travel through some really congested sections of Indian Railways. Combine that with peak winter season and its very likely that your train will be delayed significantly. It may reach the destination near midnight or even later, which could spoil your Happy New Year plans in Kolkata, if any.
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Please feel free to raise other questions as they come to mind.