CREATE TABLE table_70789 ("Model" TEXT, "Specification" TEXT, "Build date" TEXT, "Total produced" FLOAT, "Wheel arrangement" TEXT, "Prime mover" TEXT, "Power output" TEXT)

Which prime mover had a Model of rs-18?
SELECT "Prime mover" FROM table_70789 WHERE "Model" = 'rs-18'
SELECT "Prime mover" FROM "table_70789" WHERE "Model" = 'rs-18'