Worlds Largest Steam Engine! Pacing Union Pacific Big Boy No. 4014 North On Valley Sub! (4/12/26)

The water overflow from just in front of the rear cylinder on the fireman's side is the exhaust steam injector overflow port. Discharge from this port is normal when starting and stopping the injector, and when the injector switches between live steam and exhaust steam.

The diesel locomotive, while no longer necessary for PTC compliance, will continue to periodically be used where it is expedient for steam locomotive fuel conservation and assistance on long, sustained grades for power & dynamic braking.