As far as Chennai station concerned it has PERAMBUR STATION (PER) as alternative for chennai central station. Due to 2 things they preferred by pass station PERAMBUR. 1) No need to loco reversal. 2) chennai central means it has to pass through southern railways most sensitive cross over Basin bridge station (BBQ). where some times traffic congestion happens. To avoid these PERAMBUR by pass station stoppage allotted for chennai. No comments about visakhapatnam.