रेलफैनों की तो बात ही कुछ और है

That is feasible if you do the berth allocation logic for whole class like 2A at the time of charting only, else only keeping Lower berth to be allocated at time of charting will cause to much fragmentation in people's itinerary. Means if you have 4 seats, then 2 each in different bays (upper berths).
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Additionally if all berths were allocated at the time of charting only for lower classes, then there will be to heavy crowd at chart display location to search for berths and will most probably be unmanageable.

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