CREATE TABLE table_204_30 (id DECIMAL, "year built" TEXT, "works number" DECIMAL, "wheel arr." TEXT, "gauge" TEXT, "original owner" TEXT, "name" TEXT, "current location" TEXT, "notes" TEXT)

what is the difference in gauge between works numbers 541 and 542 ?
SELECT ABS((SELECT "gauge" FROM table_204_30 WHERE "works number" = 541) - (SELECT "gauge" FROM table_204_30 WHERE "works number" = 542))
SELECT ABS((SELECT "gauge" FROM "table_204_30" WHERE "works number" = 541) - (SELECT "gauge" FROM "table_204_30" WHERE "works number" = 542))