Just a small correction.
The line that was washed away in 1964 was the Pamban-Dhanushkodi line, not RMM-Dhanushkodi. RMM Dhanushkodi line was dismantled in 1927 and the main line was diverted to run directly to Dhanushkodi from Pamban.
Also, the train that got washed away was not on the Pamban bridge. It was at the outskirts of Dhanushkodi village.