Pamban Sethu - இது தான் நம்முடைய ராமர் சேது - Darnish C

I do agree that the addition of side-berths will increase the capacity, but there are a few problems here, whether we follow our existing DD coach or improve it inline with the Russian design. The dimensions of the final coach become a problem, as RDSO identified in their reports. (The reports of the studies are available online).
The main advantage for the Russian systems is the height. The Russian coaches are about 5.3m high and provide about 2.2 metres of head-room per level. Our existing DD coaches are only 4.3m high and the bottom level is only 1.8m high, which is barely sufficient for luggage space + sitting head room + head room for the upper berth....
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Our earlier side-middle berths had a little less than 2 feet of head room and they were quite cramped. With the same height as our current DD coaches, each level would be quite cramped, particularly for the upper berths, and not comfortable like their Russian counterparts. Currently, the maximum allowed height for the DD coaches is 4366mm, which has to be revised to allow more height ( and this itself infringes the guidelines for standard coaches, which had to be specially amended to allow DD coaches).
Adding side berths is also slightly problematic as the DD coaches are narrower than the normal LHB or ICF coaches ( 3040mm vs 3240 mm in normal coaches ). So, addition of side berths would make it further cramped inside. The Russian coaches also have about 15cm lesser width (around 3.1m) than our standard coaches. To have the same berth size as we do now, the corridor width will end up becoming narrower ( Currently 2 feet wide in single level coaches). With a side-berth in a DD coach with either our existing or the Russian dimensions, the corridor will be barely one foot wide and overall, the comfort levels would be very low.
Ultimately, with the present 'Maximum moving dimension' (MMD) constraints we have on IR, it is probably safe to say that having sleeper class DD coaches with both comfort and high-capacity is very difficult. IR does have designs for comfortable, high capacity coaches, but implementing that would require modifications on tracks, bridges and platforms to allow coaches with larger dimensions.