I biweekly train is possible on GR slot.
Rewa-mumbai can run on Sa and Su. and Mumbai Rewa can run on M, W, F, Su (any two days).
12187 runs M, W, F
18609 on Th
12597...
more... on Tu
so Sa and Su is available.
for return,
12188 runs Tu, Th and Sa.
(here the only thing is 12598 runs on W & 22103 on M, but both depart from Mumbai around 14:30 and reach Itarsi at 01:25, so overtaking can be possible).
so M, W, F, Su are available.
Please correct me if i am wrong.