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TRACK AND STRUCTURES CONSIDERATIONS
Train handling is affected in many ways, both directly and indirectly, by the actions of personnel other than the train and engine "crew," just as the actions of the crew can also affect the rolling stock, track, structures and their maintenance.
It is important that maintenance and design personnel understand the dynamic forces involved in train operation and their effects on the track and supporting structures. Maintenance and design personnel must also be aware of the effects of their actions, such as the placement of speed restrictions, have on the crew's ability to handle trains through areas being repaired or re-built.
It is equally important that the crew be aware of the forces expended in starting, running and stopping a train and the manner in which these forces are transmitted through the cars and their component parts (drawbar, frame, wheel, etc.) to the track and its substructure, including bridges and trestles.
A crew that understands the reasons for limitations placed on train speeds will not exceed these limits and jeopardize the safety of the train operation.
The Track and Structures considerations Section of this manual attempts to outline some of the dynamic problems without excessive use of technical terminology.
The forces expended by trains in starting, stopping, running, and negotiating curves and turnouts are transmitted from the wheel to the rail and can be divided into lateral and vertical components as illustrated in Figure 1.
If the Lateral (L) and the Vertical (V) forces generated by moving trains are not absorbed and restrained by the track, failures of some form will occur. These failures can often be predicted by studying the LN ratios (the lateral forces divided by the vertical forces) at each wheel.
Critical L/V values vary greatly, according to dynamic situations being considered; for example, L/V ratios which present a problem in low speed draft situations are not the same as those that are of concern in high speed buff situations, due to such factors as the dynamic characteristics of the vehicles, trucks and track.
The consideration of L/V ratios is important in any study of the interaction of wheel and rail in dynamic situations and must be recognized as a "time dependent occurrence" (an occurrence whose importance varies with the elapsed time.) L/V ratios are especially important in predicting track shift, wheel climb, wheel lift, and rail turnover. (See Section 3.2.4 Train Make-Up)
While the L/V ratio is important in studying track situations, the actual lateral and vertical forces are the forces which must be absorbed by the track structure supporting the train. The lateral stability of the track must be such that it will resist any tendency to shift or increase gage.
Rail wear occurs in generally predictable patterns according to the tonnage handled, the speed, the curvature and the equipment. Whenever wear greatly exceeds expected limits at a particular location, cooperative interdepartmental studies should be initiated to determine circumstances of the track, the equipment, and the train handling. SPECIAL LINES OF COMMUNICATION WITHIN THE COMPANY SHOULD BE ESTABLISHED SO THE PROBLEMS RELATING TO THE TRACK, THE EQUIPMENT, AND THE TRAIN HANDLING CAN BE MUTUALLY STUDIED AND EFFECTIVE SOLUTIONS IMPLEMENTED.
Speed restrictions are imposed on train operations for many reasons including operating, legal and technical considerations, such as limitations of the equipment, track and structures.
Most speed restrictions are imposed for the safety of the train operations so the magnitude of the forces expended will not create an unsafe condition.
The magnitude of forces expended in train operations varies with the speed of the train, with some forces increasing in proportion with the square of the speed. The greatest forces occur in slowing or stopping a train and these forces are transmitted to the track. The main train forces are longitudinal in nature which in turn can also lead to undesirable lateral forces.
The allowable speed over a section of track must not exceed the speed from which an emergency stop can be made without creating excessive stresses in the track and structures not only from longitudinal forces but also excessive lateral forces at a wheel which might cause wheel climb or rail turnover.
Speed restrictions are either permanent, recognizing the inherent design limitations of the track and equipment; or temporary, for a variety of reasons, including track or bridge repairs, emergency situations (e.g., flooding, soft roadbed, etc.), or equipment considerations recognizing the problems of a particular car or combinations of cars being handled.
THE LIMITS OF SPEED RESTRICTIONS MUST BE CAREFULLY SELECTED, CONSIDERING NOT ONLY THE SPECIFIC STRUCTURE OR SECTION OF TRACK TO BE PROTECTED BUT ALSO THE DIFFICULTY EXPERIENCED BY LOCOMOTIVE ENGINEERS IN CONTROLLING THE TRAIN AS IT APPROACHES THE RESTRICTION. IN ORDER TO ADEQUATELY PROTECT CURVES AND STRUCTURES PRIOR TO THE RESTRICTED AREA, IT MAY BE ADVISABLE TO EXTEND THE LIMITS OF THE RESTRICTION. SINCE ALL CARS IN THE TRAIN MUST BE BROUGHT THROUGH THE RESTRICTION SAFELY, THE CREW HAS THE RESPONSIBILITY TO OBSERVE THE SPEED RESTRICTION FOR THE ENTIRE LENGTH OF THEIR TRAIN UNLESS OTHERWISE SPECIFIED i.e., such as the engine speed only being limited at a crossing.
A train is made to change direction by introducing curvature into the track. The rail on the outside of the curve guides the wheel and truck by resisting its tendency to go straight thus turning the locomotive or car.
In the curve negotiation the lead wheel bears against the
ANGLE OF ATTACK
Figure 2. 3-Axle Rigid Frame Locomotive Truck
The axles can shift relative to one another due to the lateral freedom of motion built into equipment trucks. This lateral freedom allows the trailing axles to shift somewhat toward the outer rail and all axles to skew to a sharper angle of attack.
SOLID LINE WITH LIMITED LATERAL
DOTTED LINE - WITHOUT LATERAL AXLE FREEDOM
Figure 3. Standard 3-Piece Freight Car Truck
Figure 3 illustrates the relative position of the wheels and axles of a standard 3-piece freight car truck while traversing a curve with and without the effect of lateral freedom.
Wheel to rail frictional forces developed in curve negotiation are in addition to the normal forces generated in train operation and increase sharply as the degree of curvature increases. The relative magnitude of the lateral forces developed is shown in Figure 4.
As the frictional forces increase, the probability of wheel climb, rail turnover and lateral buckling of the track increases. There is a limit to the amount of curvature a truck can negotiate without excessive friction and binding. This depends on the truck wheel base, the gage of track, and the lateral freedom per axle.
EQUIPMENT MUST NOT BE PERMITTED TO OPERATE ON