Wheel slip

A condition that occurs when the acceleration forces of the locomotive exceed the friction forces between the wheels and track. This only happens on driven axels and not cars being pulled.

Cause

A locomotive (although heavy) has pretty low friction on the tracks because it's steel wheels on steel rails. This allows a considerable amount of weight being moved with comparatively little power, but it also can cause problems when not careful.

Regardless of how much weight is pulled, there is a maximum amount of traction forces that a locomotive can generate before the wheels start slipping. This amount does not change with the amount of weight you try to pull. The attached weight merely changes how quickly you accelerate with a given amount of engine power.

Effect

Wheel slip drastically reduces the friction between wheels and tracks. This means you lose almost all power to pull the train. It also causes damage to the wheels, which you will then have to pay to repair.

Indication

The diesel locomotives have an indicator lamp for wheel slip. Wheel slip on the stream train can easily be heard because of the open cabin design. The locomotive remote also has a lamp and accoustic indicator.

Resolution

Wheel slip can be resolved by either reducing the throttle, or by deploying sand. Sand increases wheel friction, but has to be refilled while generating maintenance cost.

Prevention

The best way to prevent wheel slip is to operate the throttle gently. You will eventually gain the experience and learn how far you can set the throttle for a given speed. For example, at around 57 Km/h you can fully set the throttle on the shunter and not cause wheel slip. When standing still, you can set it to around 20-25%.

Physics

Wheel slip happens when the acceleration forces exceed the friction forces between the wheels and the rails. The acceleration force gets bigger the further you set the throttle and gets smaller the faster your train goes. This is because of how induction motors work.

A second force you're fighting against is gravity when going uphill. This force gets stronger when your train gets longer and heavier. This can go to the point where it alone exceeds the friction force of the wheels, making it impossible to get further up. In this case you have to use multiple locomotives or try to make a run up and go fast enough to overcome the hill with the built up inertia of the train. When in immediate risk of stalling, deploying sand can help and may increase the wheel friction beyond the gravity forces again.

Trivia

This force balance also limits how quickly a train can stop, but it can be overcome with the train brake because all cars have brakes, giving you access to the friction forces of all axels, not only the driven ones.

The independent brakes are sometimes not strong enough and you may find that a heavy train continues to accelerate downhill with only the independent brakes applied.