Tonnage Loads of Trains

Aim - To describe the maximum load of a train under various operating conditions.

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Index

Introduction

Methodology for Calculation of Train Loading.

Useful References


Introduction

As described as described in the section called Principles for Train Movement, the amount of load that a train can haul will be determined by the balancing effect of the Tractive Force versus the Resistance Force.

In turn, the following types of factors will influence the respective sizes of the above forces, and hence will impact the train loading:

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Methodology for Calculation of Train Loading.

To ensure the most efficient and economical running of trains railway companies define tonnage (load) ratings for trains over different track sections. These ratings are to assist staff in making up consists that won't cause the locomotive to run too slowly or to stall on a hill, and block the line, as this would cause delays and cost the comapny extra costs of operation.

In setting a rating the company usually considers the following factors for each section of track:

  • Minimum speed of train operation - the speed of a train in a section will determine the length of time taken for the train to pass through the section. Adding more load will reduce the speed that a train is able to climb a gradient, and hence increase the transit time for the section. This can slow down following trains, and hence the company will usually attempt to find the best balance of load vs speed.
  • Track Gradients - the company will consider the gradients present in each track section, and usually groups the gradients as follows:
    • Ruling gradient - this is typically the steepest gradient in the section, and the train would need to climb this grade at desired speed determined by timetable requirements. Based on this gradient and the speed required, a tonnage load would be calculated for this section.
    • Helper (Bank) gradients - these are steep, often short sections of steep gradient, where it was deemed more economical to add a helper locomotive to assist the train up this grade rather then reducing the tonnage load in the section just for this grade.
    • Momentum gradients - railways are continually wanting to increase their efficiencies of operation, and hence are keen to increase the tonnage ratings. As a consequence they review the ruling gradients to see whether these can be reduced in a section by using momentum. In other words, if a train can get a "run up" at a gradient then its momentum will carry it up the gradient. This means that larger tonnages can be assigned, however it is then important to make sure that a train is not prevented from using its momentum. For example trains would not be able to be stopped at certain locations as they may not get going again, or alternatively may not be able to climb a particular gradient. In effect the use of a momentum gradient allows the company to reduce the ruling gradient for a section, and hence increase the tonnage rating for the train.
  • Track Conditions and Weather - the track conditions and weather can have an impact on the tonnage rating as well. For example in wet weather the wet track may make the locomotive more prone to slippage, and thus it is necessary to reduce the load. Icy track can reduce adhesion even further, and also when older style bearings (sliding bearings as apposed to roller bearings) were used they were more susceptible to increased resistance in cold weather.

The static friction (or starting resistance) of any given car or train is considerably greater than the kinetic friction (rolling resistance), but decreases rapidly as the speed increases up to a point between five and ten miles per hour depending upon conditions and bearing types, after which rolling resistance increases as the speed increases. For this reason the resistance of a car to motion is greater than the resistance of the car to continued motion after having been started, from which it is clear that unless some means were adopted to overcome the greater resistance of starting resistance, a locomotive would be unable to start a full tonnage train which otherwise it could successfully handle. This is done in railroad practice by the provision of slack in the coupler draft gears of the cars which in effect enables a locomotive to start a train, one car at a time.

Starting resistance being restricted to one car at a time by the operation of the slack, it becomes a very small proportion of the total resistance, and as the number of cars set in motion increases, the proportion of static resistance decreases.

Whilst the use of slack can be used to start trains of relatively large loads, it can still be somewhat difficult to start a train on a steeper gradient, as the train may not be able to be bunched to allow use of slack inthe couplers. In this instance, the locomotive is trying to overcome the full load of the train, including any associated starting resistance, and therefore the locomotive may not have enough tractive force to start the train.

To compensate for this effect, most railway companies have track designs which require stations, siding, and signals to be placed on more gentle gradients, and thus make it as easier as possible for trains to start.

Multiple locomotives used for haulage - In case of double - headed locomotives / multi-operation of locomotives, tractive effort increases with each locomotive added. Hence, haulage capacity indicated in load calculators can be summated for each of the locomotives attached to the train. Thus for example, two locomotives of the same class attached to the train, will result in the load being doubled. Similarly, for disimilar locomotives, the load that each locomotive can be hauled, can be added together to get the total load.

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Useful References

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