Which part of a train helps maintain its stability during travel?

The undercarriage is a crucial part of a train that helps maintain its stability during travel. It is designed to support the weight of the train and provides the structure necessary to keep it balanced on the tracks. This part includes various components like the wheels, axles, and other structural features that allow for smooth movement and proper alignment with the track. By distributing the weight evenly and enhancing the train's adherence to the tracks, the undercarriage plays a vital role in preventing derailments and ensuring a stable ride for both the train and its passengers.

The engine primarily provides power for movement rather than stability, while the coupler connects train cars but does not significantly influence stability. The braking system is essential for safe stopping and control, but it does not contribute to maintaining stability during travel in the same way the undercarriage does.