Not sure about exact reason. But this is my observation. Initially when extra Ac was added, they removed hcpv. Now hcpv is added back, and one general is removed from rear end. Mail is only 23 coach..I guess if hcpv is there, 23 is maximum allowed coach rather than usual 24. Just wild guess based on observation.

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Can this hcpv go with maq sfe than mail as sfe is just 22 coacher. Mail should be made 24 coacher with p7 at the earliest once electrification officially reach CAN .

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SF, 12685/86 is having CBC rake. I guess HCPV for CBC rake is not there. That is one reason for HCPV with mail.
P7 will be considered for Mail. RPM is getting new p7s. But will have to wait. Between MAS and SRR, P4 can perform equally well as P7. Small lag due to lesser acceleration is only disadvantage. But with slack between TUP and PTJ as well as OTP and SRR, this will be nullified. So as of now, p4 is ok with mail. Once fully electrified, then p7 will be better option.

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GT was getting HCPV. I guess CBC can be connected with normal coaches, I snapped a pic at Avadi where an inspection car (non CBC) with TVC mail. see the following link
/blog/post/1474494

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