SDAH NDLS Duronto: দৌঁড়ে চলো , দিল্লি চলো - PPG

Air-conditioned coaches, on a permanent basis, were introduced in 1937. The GIPR had experimented with A/C coaches in the past, most prominent being in 1914, when an experimental train used to have a refrigeration unit in the luggage van and cold fluid was circulated through pipes to keep the coaches cool. But these proved to be too cold for comfort and the public lobbied to terminate the service. It is unclear as to which train this is referring to.
Using American-based systems, IR started experimenting with A/C coaches in 1937, and by the end of the year, A/C coaches were operational on Frontier mail (1 coach, from September 1937) and the Calcutta-Bombay mail (September, 1937, most probably...
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the Imperial mail). Soon, major trains on trunk routes started getting A/C coaches on a regular basis. Trains like the GT, Howrah-Madras mail etc used to run with A/C coaches on a few days every week. All these were only A/C first class coaches.
In September, 1956, the first A/C chair car coach was put into service (third class). These coaches were first used on the 'Bi-weekly air-conditioned vestibuled' express introduced between Delhi and Calcutta from October 2nd, 1956 (Or Deluxe, renamed as Poorva sometime in the early 90's). Shortly afterwards, similar trains were introduced between Bombay-Delhi and Madras-Delhi.