Looks like the inaugural was on January 15th, 1880, but I have to double check. There were two trains from Madras to Beypore. One starting from Madras in the evening, and one in the morning. The morning train used to stop overnight at Erode and run combined with the previous day's evening train from the next morning. This train had a connecting train to Coimbatore and Mettupalayam which started in August, 1873. From January 15th, 1880, the day time train was cancelled between Podanur and Beypore, but the connecting train to Coimbatore still ran with the same timings. So, I guess that was the date from which a dedicated Madras - Mettupalayam train started.