Shabdbhedi Express: गति इसकी शब्दों को भेदती है, पूरब को पूर्वांचल से जोड़ती है - Dr. Abhishek Rai

there can be many reasons why a train was given dedicated rakes be it 2/4/5 or even more depending on
1) the distance it has to cover for an up and down journey
2) TAT at source station for maintainance
3) No other train which matches its current CC
4)...
more...

if it requires any altering in CC to give a RSA
5) May be the respective division form where the train departs does not want its train to give any nonsense RSA and ikeep it simple
Take example of 12723/12724
It used to have 4 dedicated rakes (1 as spare rake for winter times where it gets delayed due to fog in north india)
Rake 1 - As 12723 Arrived Today morning at NDLS and will return as 12724
Rake 2 - Departed NDLS yesterday as 12724 and will arrive HYB and get maintianed ( will depart tomorrow morning)
Rake 3 - Departed HYB today morning and will arrive NDLS tomorrow morning at NDLS to return as 12724
Total TAT at NDLS - 8 hours (Close to 5 hours for secondary maintainance at DEE b/w 12 PM-4:30 PM)
Total TAT at HYB - 12 hours ( Minimum 6 Hours of Primary maintainance at HYB b/w 11:30PM/12 AM-6AM)
The reason mostly is has no RSA is because due to its CC and mismatch with the timings of any other trian which originated from HYB with same CC in SC division.
Most of the times, Point 5 will be the reason behind trains having no RSA ...
This is as per my knowledge...legitimate additions/modifications are well appreciated :)
-RF