How Do Trains Get Traction?

It’s no surprise that many wonder how locomotives are able to haul such heavy loads, especially up steep gradients. Locomotives are engineering marvels, and many instances must be present in order for a locomotive to gain traction efficiently.

Trains get traction because of the immense weight of the locomotives, and the friction generated between the wheel and rail head. Furthermore, in less than ideal weather conditions, sand is sprayed on the rail head to reduce wheel slip. 

There are many factors that must come into play in order for a train to get enough traction to move heavy loads. To assist with gaining traction, many modern locomotives are equipped with traction control systems, which will control the amount of tractive effort applied to the rail head. Weight and the skill of the engineer also come into play, as more experienced locomotive engineers are more adept in getting the train rolling faster. Furthermore, the free rolling bearings on rolling-stock significantly contribute to momentum keeping the train rolling.

Todd and Jack Humphrey

Weight and Aerodynamic Drag

A modern diesel electric locomotive weighs around 400,000 lbs, and has a tractive effort of roughly 60,000 lbs, thus, there is massive force on the rail head. Some modern locomotives, such as the ES44AC, are ordered with extra weight for additional tractive effort. In short, the heavier the locomotive, the greater the tractive effort.

Aerodynamic drag is also a key factor in how trains are able to gain tractive force. Trains have very little friction between the wheel and the rail-head, thus, allowing trains to move at quicker speeds on level ground than cars, which have rubber tires, thus, aerodynamic drag is greatly increased. Contrary to popular belief, the surface of the wheel that makes contact with the rail-head is merely the size of a small coin. When the train reaches its maximum speed, the aerodynamic drag is equal to the tractive force. Although trains can operate efficiently on level surfaces, traction is greatly affected by oil substances on the tracks caused by crushed leaves and other slippery substances, however, appropriate precautions are taken.

However, when a train begins to climb a significant gradient, the weight of the locomotive produces drag and slows it down. This is where a car with tires has an advantage, as it creates more friction, and allows the car to crest the gradient without issue. To combat this lack of traction on grades, locomotives are coupled to the rear of the train in order to push it over the hill.

Todd and Jack Humphrey

Different Types of Locomotives

The amount of tractive effort supplied by a locomotive depends on the locomotive. For example, back in the days of steam, wheel slip was much more prominent than in the current age of the diesel and electric locomotive. This is due in part that various technologies are installed on newer locomotives to assist in mitigating wheel slip. Steam locomotives encountered wheel slip quite frequently, which was caused by the lack of ample weight on the large driving wheels, leading to adhesion issues.

One of the main ways trains can gain adhesion is to add multiple locomotives to a consist, which is quite a common practice in the modern era of railroading. This adds more horsepower to the train and allows for increased adhesion. In order to gain traction when commandeering a heavy train, multiple locomotives are utilized to generate enough power. This is performed by a series of electrical and air systems connected to the locomotive via various hoses.

With modern locomotives, lights in the cab will illuminate to alert the crew to wheel slip. In most modern locomotives, sanding begins automatically when wheel slip occurs. Furthermore, many systems are implicated to further reduce the tractive effort on the locomotive in order to mitigate impending wheel slip. Additionally, various modern locomotives are built with additional weight added, for example, CSX’s ES44AH, the “H” denoting high adhesion. This high adhesion allows the locomotives to have exceptional slow speed control, made possible through additional weight and TM3 adhesion control software. This software allows the pounds-force to be increased by 3,000 lbs from 30,000 lbs to 33,000 lbs.

Wheel Slip and Traction Control Systems

Wheel slip often occurs when too much power is applied to the wheels by the engineer. For example, if a train begins moving from a stop, the engineer notches up the throttle gradually, rather than applying high power causing the wheels to lose traction. A locomotive’s power and the engineer’s skill is often tested the most when starting from a complete stop.

The engineer must know how much power to apply to both avoid wheel slip and coupler damage. Wheel slip is both detrimental to the locomotive traction systems and track infrastructure, and must be dealt with accordingly. If locomotive wheel slip is not dealt with accordingly, the locomotive spinning its wheels could grind down the steel, creating a massive gully in the track. If this type of infrastructure damage occurs, a speed restriction could be mandated because the metal has been weakened. Sometimes damage is so severe, that the piece of rail must be cut out and replaced.

Modern locomotives are equipped with various traction control systems that allow them to reduce or eliminate wheel slip. These types of technology are oftentimes used to govern how much power is sent to the wheels in order to avoid wheel slip and retain traction.

Todd and Jack Humphrey

Although too much wheel slip could cause inevitable damage to both the locomotive and the railroad infrastructure, some wheel slip is actually advantageous. Many modern locomotives are equipped with a system called CREEP, which regulates how much wheel slip the locomotive experiences. This small amount of wheel slip actually increases the locomotive’s adhesion above the normal 25%. This system is controlled by computers and other facets of technology that needs to work in unison in order for this feature to operate correctly.

These various types of wheel slip controls date back to the 1960s, when Electro-Motive Division (EMD) began experimenting with their Individual Detection and Correction (IDAC) system, which were first offered in the six axle SD45. Although this technology is effective, it must be maintained regularly in order to work properly.

Although these various systems are effective in controlling wheel slip, engineer skill is paramount in maintaining traction. Engineers must know how to control the slack in the couplers in order to maintain control of the train, which prevents breaking coupler knuckles and draft gear.

Inclement Weather Conditions

Inclement weather conditions hinder railroad’s efforts to operate efficiently, as wet and snowy weather effects operations immensely. One of the most prominent offenders hindering operations are leaves on the track, predominantly in the fall. When leaves fall from the trees and land on railroad tracks, they are crushed by passing trains, causing the leaves to exert various oils, which are slippery, and often cause delays. In fact, many railroad will operate special trains to clear the leaves and oils from the tracks, for example, NJ Transit’s “Aqua Train”.

Snow and ice greatly hinder railroad operations as well, as it causes the rails to be incredibly slick, and in some instances, impassible. Various railroads combat these conditions with snow jets, which is a jet engine powered blower. This blower operates as a powerful snow blower and de-icer, clearing tracks and switches of ice and other debris.



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