original: since 1987 , the heavy rail passenger mass rapid transit -lrb- mrt -rrb- metro system has been in operation .
simplifications: since 1987 , the heavy rail passenger mass rapid transit -lrb- mrt -rrb- metro system has been used . since 1987 , the heavy rail passenger mass rapid transit -lrb- mrt -rrb- metro system has been in operation . the mass rapid transit -lrb- mrt -rrb- has been a heavy rail passenger system since 1987 . the heavy rail passenger mass rapid transit -lrb- mrt -rrb- metro system has been in operation since 1987 . since 1987 , the big rail passenger mass rapid transit -lrb- mrt -rrb- metro system has been working . since 1987 , the heavy rail passenger mass rapid transit -lrb- mrt -rrb- metro system has been in operation . since 1987 , the heavy rail passenger mass rapid translt -lrb- mrt -rrb- metro system has been used . the mass rapid transit -lrb- mrt -rrb- metro has been in operation since 1987 .