The Metropolitan Rapid Transit or MRT is a mass rapid transit system serving the Bangkok Metropolitan Region in Thailand. The MRT system comprises two heavy rail lines, with a further three lines (one heavy rail line and two monorail lines) currently under construction and due to open in 2021. The MRT Blue Line, officially the Chaloem Ratchamongkhon Line, between Hua Lamphong and Bang Sue was the first to open in 2004 as Bangkok’s second metro system. The MRT Blue line is officially known in Thai as rotfaifa mahanakhon or “metropolitan electric train”, but it is more commonly called rotfai taidin literally, “underground train”。