Increasing speed through turnouts: a FRA-sponsored study looks at low-cost means to increase safe speeds through turnouts, by way of a retrofit or upgrade

Railway Track and Structures,  July, 2004  by Clifford S. Bonaventura,  Allan M. Zarembski,  Joseph W. Palese,  Donald R. Holfeld

• E-mail
• Print
• Link

Turnouts represent a discontinuity in the track structure, made necessary by the physical requirements of moving a rail vehicle from one track to another. The nature of the turnout generates high levels of force and acceleration, particularly in the areas of the switch point and frog. These forces are illustrated in Figure 1, which shows the variation in lateral force from "optimum." Since these high force levels cause excess wear and tear on the turnout, as well as an increased "risk" of a wheel climb derailment, the goal of any improved turnout design is to reduce these high force levels and bring them closer to a uniform level, as illustrated by the reference optimum line in Figure 1.

[FIGURE 1 OMITTED]

Because of these discontinuities, and associated high force levels, turnouts usually require a speed restriction. For example, the highest diverging speed permitted on conventional AREMA-design No. 20 turnouts is of the order of 45 miles per hour. To obtain a higher diverging speed, it is usually necessary to completely replace an existing turnout with a new one, generally of much greater length to allow for a more gradual change in force levels. This can require additional track space that may not be available due to physical constraints. It also may require a complete turnout replacement which can be extremely costly.

As a result, interest has grown in finding a low-cost means of increasing permissible speeds through turnouts that are compatible with the conventional AREMA standard designs. This article presents the results of a Federal Railroad Administration-sponsored study, "Investigation of Low-Cost Techniques for Increasing Speeds Through Special Trackwork." (1) The objective of this study was to identify potential turnout modification techniques that may be used to retrofit or upgrade existing turnouts for the purpose of increasing operating speeds. The results of this study led to the development of a new-design turnout.

Conceptual approach

One criteria for the new turnout design was the requirement to maintain the existing turnout length, i.e., one that would not require any change in the overall length of the turnout. Thus, the proposed turnout modifications could not alter the existing turnout lead length or the existing frog angle.

Since maximum allowable track speed through the diverging leg of the turnout is, in part, based on the amount of lateral force applied between the wheel and the rail, any improved high-speed turnout design should reduce and minimize the amount of lateral force (or force ratio, L/V) through the turnout, and, in particular, the peak or maximum lateral force that occurs. As can be seen in Figure 1, for a standard AREMA turnout design, the largest force is usually generated at the point of switch due to the sudden change in angle between the straight route and the switch rail. The heel of switch and the frog are other regions where the lateral forces typically increase.

[FIGURES 2-3 OMITTED]

Reducing these forces can result in an increase in the allowable speed. However, if the overall length of the turnout is unchanged, the average turning force throughout the turnout cannot be changed, regardless of the design modification. The best approach, then, is to design (or redesign) the turnout so that the turning force is uniformly distributed throughout the entire length of the turnout. The resulting uniform distribution would result in the highest-possible speed limit. Since a major constraint was that no change in the lead length of the turnout was permitted, conventional tangential geometry turnouts, which feature near-zero switch angles and more uniform geometry, were ruled out. (2)

Another consideration was that the design modifications were to be of relatively-low cost, which eliminated moveable point frogs.

After investigation of several alternatives, it was determined that the most viable way to increase turnout speeds was to eliminate the lateral force spikes and introduce a uniform lateral force transition throughout the entire length of the turnout.

Geometry optimization

Due to its prevalence in the industry, the No. 20 AREMA turnout with straight switch points was selected as the focus for the geometry optimization. The resulting analysis of increasing turnout speed by optimizing the degree of curvature (for use in the [V.sub.max] formula) was performed, based on the turning angle, i.e., the frog angle. However, as noted, the fixed frog angle was only one of two primary geometrical constraints identified earlier. The other constraint was the need to make little or no modification to the actual lead length of the turnout.

As a consequence of these additional constraints, optimization of the turnout geometry could not be achieved by a simple analysis of the turning angle and the distance along the track. Instead, it was necessary to generate the geometrical equations describing the layout of the diverging route of a turnout. The resulting optimization process was, therefore, conducted for two different geometrical sequences.