It’s an MRT (Mass Rapid Transit). Since we always pronounce the abbreviation MRT as separate letters, we use the article "an".

It’s an MRT (Mass Rapid Transit), so we say "an MRT line", "an MRT station", "an MRT system", etc. Since we always pronounce the abbreviation MRT as separate letters, and the M begins with a vowel sound, we use the article "an".

There are plans to build an MRT system soon.

There are plans to build a MRT system soon.

Be aware that the rule for using a or an is based on pronunciation—not spelling.

An MRT station was chosen as a case study.

And we always say an M (not "a M").

You can use an MRT line that connects to the airport.

Follow the same strategy with other abbreviations that begin with M and are pronounced as separate letters; for example, we say an MBA (Master of Business Administration), an MD (Doctor of Medicine), or an MIT (Massachusetts Institute of Technology).

Is she an MIT student?