रेलफैनों की तो बात ही कुछ और है

51/52 reach early and depart early (at/from both the ends) while 53/54 reaches after it and also departs after it (at/from both the ends) .
In this situation, RSA would mean that early reaching rake will depart late and late reaching rake will depart early which shows - no purpose.
(In case of 19115/16, 19131/32 the rakes are logically shared at Bandra Terminus. 19116 reaches late but departs early hence as 19115, the rake coming as 19132 is used to avoid any possible delay and also to maintain the rakes. Yet, the...
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structure of both the trains is THE SAME.)
On the other count, B9 in 51/52 is in between the rakes and attaching / detaching on daily basis may not be so easy.
(Jaipur-Nagda-Kota-Jaipur rakes are shared for Jaipur-Bayana-Jaipur. The former has AC class while the later does not have. Yet, the AC coaches are not detached but kept locked and those run with the rake empty. You will never find B9 in 53/54.)
The coaches of 19251/52 are having plates indicating New Delhi while the coaches of 12953/54 indicate Nizamuddin. Changing plates of all the coaches of both the trains on daily basis is not expected.
To remove any misunderstanding, I wish to add here that as per the need, coaches of both the trains are used from time to time , to be attached to other rakes after the detached maintenance.