Had a similar problem with a Lionel 0-6-0 tank engine on display duty at a convention using O27 curves. Had to remove the centering spring in the field, which also holds the coupler up. The little indefinite flew off to parts unknown. The following field expedient repair is not recommended for the faint of heart.
As they say in German, "Zum Höllen, es lauft!"
Apologies for the cluttered backdrop, it's ZW disemboweling season.
Mitch