CREATE TABLE table_63992 ("Class" TEXT, "Wheel arrangement" TEXT, "Fleet number ( s ) " TEXT, "Manufacturer" TEXT, "Serial numbers" TEXT, "Year made" TEXT, "Quantity made" TEXT, "Quantity preserved" TEXT, "Year ( s )  retired" TEXT)

What is the fleet number for the model with Serial number 68056?
SELECT "Fleet number(s)" FROM table_63992 WHERE "Serial numbers" = '68056'
SELECT "Fleet number(s)" FROM "table_63992" WHERE "Serial numbers" = '68056'