I think they extended DM to HDB to free up parking space from NJP. During the pre-covid days, I frequently saw the Darjeeling Mail rake being taken towards SGUT/SGUJ from NJP after the termination of its journey.
I hope NJP retains its General quota as HDB, JPG, and, NJP are in the KIR division, and then it will be a win-win for all.
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Pre-partition, DM used to run till Siliguri Town, and connection to the DHR was available from SGUT only. The present NJP station came up in 1960, a station on the Assam Link Project (the new faster line between Kishanganj and Fakiragram). The present Siliguri Jn, earlier an MG station also came up post-partition in 1949. The original northern terminal of Darjeeling Mail, Siliguri Town came up in 1880.