Greg, that sounds like the 2-4-2T tank engine, "Lyn" and the black version, which I also took apart once. They have a split axle. The original Columbia tender engine does not.I just got hold of an exploded parts diagram, but it shows the axle as a split axle, 2 halves connecting to the center gear, which is much larger, so apparently there is more than one version.
For a gear that size? You guys do have a hard time.