A quick disclaimer- The present day Darjeeling mail and the pre-partition Darjeeling mail are completely different trains.
The present day Darjeeling mail is a renamed version of the North Bank express that was introduced after independence.
That said, before partition, the Darjeeling mail ran from Sealdah to Siliguri (the present day Siliguri Town station) via Ranaghat, Ishurdi, Parbatipur and Haldibari.