CREATE TABLE table_204_816 (id DECIMAL, "locomotive" TEXT, "named" TEXT, "serial no" TEXT, "entered service" TEXT, "gauge" TEXT, "livery" TEXT)

what locomotive is just previous to bl29 ?
SELECT "locomotive" FROM table_204_816 WHERE id = (SELECT id FROM table_204_816 WHERE "locomotive" = 'bl29') - 1
SELECT "locomotive" FROM "table_204_816" WHERE "id" = (SELECT "id" FROM "table_204_816" WHERE "locomotive" = 'bl29') - 1