Practical Engineering - Why Locomotives Don't Have Tires

Short Summary

Main recommendation: Locomotives do not need tires because they rely on steel-on-steel adhesion between wheel and rail, where friction, not rubber, provides the traction needed to pull trains. Rubber would offer more grip but isn't durable enough for freight's heavy loads and long mileage, so engineers manage traction through locomotive weight, friction control, and creep control systems instead.

Key Points

• Traction is limited by friction at the wheel-rail contact patch, roughly the size of a US dime (about 2 to 3 square centimeters, or half a square inch), so the usable pulling force depends on that small area.
• Diesel-electric locomotives face an inverse relationship between speed and pulling force, and friction at slow speeds (the adhesion limit) can prevent the engine's full power from being used.
• Friction depends on two factors: the normal force (essentially the locomotive's weight) and a coefficient of friction that is affected by contaminants such as oil, grease, rust, rain, and leaves.
• Wheels under traction creep, meaning they spin slightly faster than the train moves because the contact area deforms, and maximum usable traction occurs at a peak point on the traction-versus-creep curve.
• Modern creep control systems monitor each wheel and adjust tractive force to stay near that peak, improving use of available power and reducing fuel and wear.

Details Worth Noting

• Concrete instructions: No step-by-step instructions are given for a viewer to follow. The video does describe the calculation engineers use: divide the steepest section's required tractive effort by the dispatchable adhesion (the friction coefficient that can be relied on for the locomotive and conditions), giving the total weight needed. Examples cited are about 30% for a modern locomotive on dry rail and 18% for an older model on a frosty morning. Practical traction-improvement methods mentioned include sanding the rails (historically via sandboxes), and cleaning rails with air, water jets, chemical mixtures, or lasers.
• Large freight trains often use two, three, four, or more locomotives because there are limits on weight per wheel before tracks or bridges fail.
• Rubber tires are used on some passenger rail vehicles, and some locomotives have removable steel "tires," but freight pit stops are expensive, so durable steel-on-steel is preferred.
• Stick-slip, where friction alternates between sticking and sliding, can cause rail corrugation and noise and was a notorious problem for steam locomotives.
• The video cites a Federal Railroad Administration report calling friction the "last frontier" of vehicle/track interaction.

Bottom Line

Locomotive traction is a managed balance of weight and friction rather than a matter of better tires, and modern control systems squeeze extra pulling power from that steel contact.