original: Since 1987, the heavy rail passenger Mass Rapid Transit (MRT) metro system has been in operation.
simplifications: The heavy rail passenger Mass Rapid Transit (MRT) metro system has been working since 1987. Mass Rapid Transit (MRT) metro has been working since 1987. The Mass Rapid Transit system has been operable since 1987. The Mass Rapid Transit (MRT) metro system has been in operation since 1987. Since 1987, the Mass Rapid Transit (MRT) metro system has been operating. Since 1987, the heavy rail passenger Mass Rapid Transit (MRT) metro system has run. The Mass Rapid Transit metro system has been running since 1987. Since 1987, the Mass Rapid Transit metro system has been operating. Since 1987, the rail system, Mass Rapid Transit (MRT) has been in operation. The heavy rail passenger Mass Rapid Transit (MRT) metro system has been in use since 1987.