Mumbai and Mumbai based rajs are, so as to say, kinda weird. Their user base is rather varied, so as to say, and has lead to success of all 3 rajdhani.
1)12951 and 12953: these raj derive their user base not just from Mumbai but also from Surat, which is arguably amongst the top 10 cities in India. The passenger base from both Mumbai and surat is bound to Delhi, so intermediate booking does not apply. Another unique point from which they derive passengers is Kota, which is a major entrance coaching hub and has many people from and around Mumbai studying there, leaving to significant people traveling that way
2)...
more... 22221: Mumbai suffers from a major problem of having North South connectivity but not east west one. As a result it is inconvenient for people staying on the eastern side of Mumbai (where, owing to the geography of Mumbai a significant chunk actually stays) to travel towards western line. CR bound raj derives a good chunk of passengers from this, along with deriving passengers from Nasik and Jalgaon who earlier had to rely on much slower Mangala SFE or Punjab mail to reach delhi.
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The user base of these trains doesn't have a significant overlap as you might think.