CREATE TABLE table_14002 ("City" TEXT, "System" TEXT, "Category" TEXT, "Daily ridership" FLOAT, "As of" TEXT, "Opened" FLOAT, "Stations" TEXT)

Which Opened has a Category of diesel light rail?
SELECT MAX("Opened") FROM table_14002 WHERE "Category" = 'diesel light rail'
SELECT MAX("Opened") FROM "table_14002" WHERE "Category" = 'diesel light rail'