रेलफैनों को आता है ज़िन्दगी का मज़ा लेना

It is not, sir.
12321/22 was historically called the 'Bombay Calcutta mail via Jabalpur' and has been in operation since 1867 (as a special train between ALD-JBP between 1867-1870, as slip coaches on the Calcutta-Delhi-Peshawar mail till 1891 and as a standalone train since then). this was the 'regular' mail train.
The Imperial Indian mail was a special train operated exclusively for First class passengers (mainly targeted at European and American tourists and visitors, though a lot of rich Indians also used it). It was...
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a weekly train that was started in November, 1926. The entire train only carried 32 First class passengers and their servants. It was the fastest train in the country and was not listed in the timetables.
The Imperial mail also took the same route as the regular mail, just bypassing ALD. During the 2nd world war, the Imperial mail was discontinued.
But the Imperial mail was a much more famous train than the regular train and so many rail fans do not know about the history of the regular train and have mistaken it to be the same as the Imperial mail. In reality, both were completely different trains.