This is one ride I would highly recommend. The train leaves Chennai at 830 in the evening, and reaches Rameshwaram by 1 the next afternoon. Despite the length of the journey, the last hour will be one of the most spectacular rides experiencable!
90min out of Rameshwaram you glimpse the ocean on one side. The smell of stale fish is quite prevalent, especially due to the recent floods and hurricane. As you keep going the sea becomes more constant, until it is right next to the tracks. You could jump from the train into the Bay of Bengal. Rameshawaram is an island connected to the mainland by initially rail and then some years later, by road. As you reach the final point of the mainland, clear blue water apprears on both sides - the train is still on solid land, but there's space for nothing else. Mainland India thins out and before you know it, you're on a bridge. Its an awesome sight and no words can do justice to it. Be there to experience it.