Howrah–Mumbai CSMT Superfast Mail (via Gaya), with train numbers 12321 and 12322 also known as Kolkata (Calcutta) Mail during the pre-independence era, is a daily superfast train running between two metro cities Kolkata, (Howrah station) in West Bengal and Mumbai CSMT in Maharashtra.

[1] The train would initially cross the Thull Ghat from Bombay towards Jabalpur and Allahabad, where it would reverse and travel towards Patna to Howrah.

The train was initially run by East Indian Railway as a passenger service with mail coaches attached.

From the 1890s, to address the quick movement of mail between Bombay and Calcutta, several postal special trains were run by GIPR and EIR.

Difference was while the Calcutta Mail would start from Victoria Terminus, the Imperial Indian Mail would start from Ballard Pier Mole railway station because it provided a connection to the British tourists arriving by ships towards Calcutta, the then capital of British India.

It is hauled by a Howrah Loco Shed based head on generation equipped WAP-7 electric locomotive on its entire journey.