The Great Gold Robbery, Part 4 - S1.E16

The train full of gold is about to crash into another train, with Riff-Raff's plan going just as he expected. However, Underdog has just rescued Sweet Polly and manages to make it in time, stopping the two trains before a major derailment occurs. Underdog then chases Riff-Raff throughout the station winds up catching Riff-Raff, and all is well once again.