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"On the other hand Calcutta - Siliguri passenger service was started just after discontinue of Darjeeling mail which was known as North Bengal link which is later renamed as DARJEELING MAIL .. So the North Bengal link exp is not other than the diverted version of old Darjeeling Mail."
- No sir, it is not.
Both are completely different trains.
1) North Bengal...
more...

express only ran till Sakrigali ghat. There was a connecting MG train that ran from Manihari ghat to Amingaon (not Siliguri).
2) The Calcutta- Sahibganj/Sakrigali ghat train was present even when the old Darjeeling mail via Bangladesh was running. It is just that it was not a very important train at that time as the Darjeeling mail was a more important one.
Once the mail was canceled, the Calcutta-Sakrigali ghat train became the only train to North Bengal form Calcutta. So, it was sped up, and reintroduced with a faster schedule as the 'North Bengal express'.
In fact there were two daily trains from Calcutta to Sakrigali ghat/Sahibganj. Both trains had connecting steamers to Manihari ghat and at Manihari ghat there were 2 connecting MG trains running till Amingaon. One was the MG North Bengal express and the other was the MG Assam link express.
The North Bengal express is unrelated to the old Darjeeling mail. Both were completely different trains.