Sir think in total there are 10 lower seats
and while booking 15 applied for lower berth all 60 plus
what next????
If one books ticket in advance [90 days] for all 90 days he/she will be in tension about which seat he/she will get..
Hence...
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ARP is 90 days which is fixed for all so plan up your journey in advance to get better seat
Regard's
Have a nice day Sir

Your idea sounds very useful for aged persons and females. But just think, if your idea is implemented in totality, what will be the result.
1-passengers of one family travelling in same boggey will get scattered, due to 'age' factor.
2-Knowing the berth number at last moment means, you are in dark about your berth no. Chart is not displayed always in time, and if displayed it is beyond reach of all. Location of chart and aged people standing at platform where train is arriving is different and distant. Instead of helping 'aged'...
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persons, your suggestion will tire them out such that they will feel harassed.
3-Your number not printed on ticket will create confusion and fight among each other cotravelers. Any one can write no, mannually and any one can claim your berth, as chart copy is not carried and given to each passenger.
4-Your suggestion is just outlandish, and impractical. Even in air travel, seat numbers are allotted at the time of booking.
5-As other have pointed out , that to receive you at destination, the family, friends too will be in dark where to stand ,which compartment you are .
6-Use of boggey/ coach number of arriving train on platfom displayed on computer display too will loose its relevance if your suggestion is implemented.