CREATE TABLE table_40986 ("Class" FLOAT, "Date" CLOB, "Builder" CLOB, "No. built" FLOAT, "1919 nos." CLOB, "LMS Class" CLOB, "LMS nos." CLOB)

On which date was the class 157?
SELECT "Date" FROM table_40986 WHERE "Class" = '157'
SELECT "Date" FROM "table_40986" WHERE "Class" = '157'