"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...
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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.

Sir , There was no train named North Bengal Exp ran B/W Sakrigali Ghat & Calcutta before independence..
Even Sealdah has no train which ran via Sahibganj loop.
No one can shown such trains.
Even katihar was connected via Lalgola - Godagari ghat - Malda Court. .
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If there was 2 link train to Amingaon ,, one is converted to kamrup express then why other train discontinued ??
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If NORTH BENGAL LINK exp ran upto AMINgaon then why new DM terminated at NJP ?

Please read my earlier post carefully, sir. I didn't say that the North Bengal express existed before independence. I said that there was a train running between Calcutta and Sahibganj/Sakrigali after independence, which was sped up and introduced as North Bengal express after the cancellation of Darjeeling mail.
During the partition, cross border services did not operate for a long time. During this time (1947-48), the entire north east was cut off from Calcutta. So, passengers relied on the following chain of trains -
1) Calcutta (Howrah) to Sahibganj or Sakrigali ghat...
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by Broad gauge (by a passenger or the successor of the loop express)
2) Manihari ghat to Kishanganj by metre gauge.
3) Kishanganj to Siliguri by the narrow gauge DHR extension.
Even before partition, there was a Howrah-Kiul passenger (running since 1927 at least based on my EIR timetable of 1927) , and a loop express between Howrah and Danapur via the Sahibgunge loop that were used by passengers to get to Sakrigali ghat and cross the Ganga by ferry. These were running in 1943 ( you can see the timetable on the IRFCA site).
These passenger services were reorganized after independence so that people from Calcutta had access to North Bengal while the problems of the partition were sorted out. One of the reorganized services was sped up, converted into a fast train and branded as the 'North Bengal' express.

The MG portion of the North Bengal express ran as an express between Manihari ghat and Siliguri and thereafter as a slow train till Amingaon with many stops.
In 1964, the Farakka extension was constructed and the North Bengal express was diverted to terminate at Farakka. At the same time, the Old Malda- Chamagram line was constructed on the other bank. On 1st April 1964, the broad gauge line from Khejuria ghat to New Jalpaiguri was opened and the old MG North Bengal Link express was diverted to start from Khejuria ghat (near Malda) instead of Manihari ghat. This new BG avatar was renamed as the Darjeeling mail.
The...
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meter gauge Assam link express continued to operate from Manihari ghat.
Note that in 1964, the BG line terminated at NJP. The NJP-Fakiragram-Amingaon section was still MG. So, the new BG Darjeeling mail terminated at NJP. This soon became a very popular train. So, when the BG line between NJP and New Bongaigaon was opened in 1965, the Darjeeling mail was not extended.
As far as trains to Guwahati were concerned, it still had the Assam mail (with a connection to Delhi) and later the Kamrup express.