SELECT "Serial numbers" FROM table_63991 WHERE "Class" = '4-8-4 — oooooooo — northern'
What are the serial numbers for the locomotives of Class 4-8-4 oooooooo northern?
CREATE TABLE table_63991 (
    "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
)