The main reason a train wheel stays on the track is because of the rim.

The rim refers to a disc on the inside of a train wheel that is larger than the wheel.

It is tightly stuck in the middle of the rail, allowing the wheel to always run on the track without derailing. This not only ensures that the wheels always run on the track, but can also be used to control the direction of the train. When the train turns, the bogie rotates so that the outer rail is higher than the inner rail, preventing the train from rolling over due to excessive centrifugal force when turning.

In addition, train wheels are tapered so that the contact surface of the wheel on the track becomes a point.

This design allows the wheels to automatically adjust their position during travel so that they remain centered on the track. The tapered wheels allow the outside wheels to travel a longer distance on the track than the inside wheels when driving through a curve, allowing the car to remain stable during turns.

Other factors such as suspension, track design, and car connections also help train wheels stay on the track.

The suspension system includes components such as springs and shock absorbers, which help balance the different forces experienced by the car during travel, thereby reducing shaking and bumps. The track design needs to be kept flat and the curvature of the curves needs to be appropriate. To improve stability and comfort, joints between rails are often ground and maintained. The connection between carriages is also a factor affecting stability. Buffers and tie rods are usually used to connect the carriages to alleviate the impact between carriages.

To sum up, keeping train wheels on the track mainly depends on the function of the wheel rim and other factors such as suspension system, track design and car connection. These designs and technologies ensure the stability and safety of trains and improve transportation efficiency.