I have another suggestion.
Why not shift DD to Tambaram? Can start by 0700 at TBM and go via Egmore, Perambur and reach SBC by 1330. Can return at 1430 and reach TBM by 2100. There is no point in running 4 daytime trains within a gap of 2 hours from MAS.
And more important, run VB as MAS SBC.Start at MAS at 0730 to reach SBC by 1140, leave SBC by 1300 to reach MAS by 1715. This will be useful for SBC MAS sector passengers. As...
more... of now, both Shatabdi and VB reach Chennai reach late evening and reaching to suburbs is tedious.