Firstly, the name 'Boat mail' was an unofficial name. The timetables of that era do not list any train of this name, but all general news articles referred to the train as Boat mail.
First, the train to Tuticorin, which started on November 6th, 1896, was referred to as the 'Boat mail'. After the new link via Dhanushkodi was opened, a new train was started to connect to the ferry at Dhanushkodi, and the old 'Boat mail' train was short-terminated between Tuticorin and Trichy. The daily boat service from Tuticorin was also discontinued from August, 1914.
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more... The original Tuticorin-bound Boat mail's present day descendant is the Pearl City express.