CREATE TABLE table_204_637 (id DECIMAL, "type" TEXT, "gauge" TEXT, "railway" TEXT, "works no." TEXT, "year" TEXT, "builder" TEXT)

builder that has the most experience
SELECT "builder" FROM table_204_637 GROUP BY "builder" ORDER BY COUNT(*) DESC LIMIT 1
SELECT "builder" FROM "table_204_637" GROUP BY "builder" ORDER BY COUNT(*) DESC NULLS LAST LIMIT 1