Both Express and Mail were SR trains earlier. Only Dadar SF was a CR train. A decade back these were moved to CR for maintenance, not sure why.
It would be better that MS SA express gets a dedicated rake, as it could become a 24 cars rake. Now due to RSA, it is only 18 cars rake, and the departure as MS-SA Express is mostly stinking due to immediate departure from MS.
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Also in the morning, the Mail has only platform maintenance at MS. I guess even the water is topped up at RU only (correct me if I am wrong).
One more point, For Maintenance Gopalswamy Yard (MS) does a better job compared to Basin Bridge (MAS)
But one is common. Be it SR or CR, both these trains get the least priority with respect to better rake allocation.