हम हवाई जहाज़ उड़ाने वाले, फिर भी फ़िदा हैं, पटरी में दौड़ने वालों पे. - SK Mangla

Refer - (Parliamentary Debates: Official Report, Volume 46, Issues 19-26) -: click here ( Only snippet views available - Can request Google for allowing access)
And also this wonderful travelogue/report from Mr. Dipak Raychaudhuri, a veteran Railfan. - click here
While this train shares it's name with the prestigious Darjeeling mail that operated till the 1950's, this was a completely new service. Note that the date updated here is only the date from which this train started running with the name 'Darjeeling...
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mail'. Prior to this, it was running as the 'North Bengal express', initially running between Sealdah and Sakrigali Ghat and later, at the time of the change in name, between Sealdah and Farakka Ghat. In 1964, the service was renamed to 'Darjeeling mail'.
Initially (around 1956) there were proposals to rename the Avadh-Tirhut mail running on the meter gauge between Kanpur and siliguri as the Darjeeling mail, but the change was not carried out.