Trying to stylise the letter M to be distinctive is pretty circuitous - just use an icon of a train! You remove the problem almost entirely, and get a symbol which is easily recognisable even to people who don’t speak the local language.

In Sydney, in the last 10 years our icons have been replaced by stylised letters. T stands for train (not Taxi or Tram), except when it doesn’t and M stands for Metro (which everyone still just calls the train), L stands for light rail (which everyone calls Trams)… it was so much more clear when there were just icons.