Table: npr_locomotives
Columns: class
wheel_arrangement
fleet_number_s
manufacturer
serial_numbers
year_made
quantity_made
quantity_preserved
year_s_retired

Question: What are the serial numbers for the locomotives of Class 4-8-4 — oooooooo — northern?
SQL: SELECT "serial_numbers" FROM npr_locomotives WHERE "class" = '4-8-4 — oooooooo — northern'

Question: What is the fleet number for the model with Serial number 68056?
SQL: SELECT "fleet_number_s" FROM npr_locomotives WHERE "serial_numbers" = '68056'

Question: What is the serial number of the model made in 1930?
SQL: SELECT "serial_numbers" FROM npr_locomotives WHERE "year_made" = '1930'

Question: What is the wheel arrangement for the model that was retired in 1955?
SQL: SELECT "wheel_arrangement" FROM npr_locomotives WHERE "year_s_retired" = '1955'