ERN and ERS comes on the same line, just about 4 kms away. The difference is that before reaching ERS and soon after departing from ERN, there is a direct track towards kottayam - trivandrum, to your left. Trains like Kerala express and venad express which also goes to ERS have to undergo engine reversal at ERS, and take another track from ERS to this main track which is going from ERN to kottayam. Trains which are going to trivandrum via alappuzha doesnt have these probs, they go straight from ERS to alaphuzha via kayamkulam and join the main line there to trivandrum. There is no need for engine reversal at ERS for trains going to alappuzha. Most of the newly introduced trains follow this route. This track structure is like a triangle - one track going to kottayam from ERN, the other ERN - ERS track, and the...
more... third, ERS - kottayam track.