Switches for Good Performance and Long Service Life

The Transportation Technology Center Inc. (TTCI) conducted a study of transit switch designs for the Transit Cooperative Research Program (TCRP) to determine which design features contribute to good performance in transit service. TCRP members observed that often the best performing switches and turnouts were some of the oldest designs.

The project was conducted to harvest the wisdom of the early designs and to retain the features that contribute to good performance in current and future designs. The Technical Advisory Group (TAG) selected a good performing switch design for the project. The tangential, spiral alignment was measured in a No. 8 turnout built in 1926 on the Southeastern Pennsylvania Transportation Authority (SEPTA). The performance of the turnout was modeled using the vehicle that typically operates over the turnout; i.e., a SEPTA B-IV car (length = 68 feet).

A series of parametric studies, in which switch/turnout properties were varied, was conducted. A comparison was made to an AREMA-style nontangential, circular curve alignment turnout. The studies showed that the SEPTA switch turnout is predicted to outperform the replacement switches significantly for the range of operations used (i.e., 5 to 15 mph). Key factors in the better performance were the tangential switch entry and the spiral alignment. Other features, such as a cast austenitic manganese steel (AMS) switch point, do not improve performance over modern rail steels.

Background

Transit switch designs in the early 1900s were quite sophisticated by today’s standards, with features that have been rediscovered and are being implemented into modern high-speed switch designs. These switches gave excellent service for many years, often outliving the lower cost but simpler replacement designs that followed. Many of the designs became obsolete as most were unique to a particular transit system, and, in some cases, when casting patterns were no longer available.

TCRP members observed that often the best performing switches on their systems were the oldest switches. The original tangential and spiral geometry switches often had service lives of 50 years or more. Additionally, these switches would outlive their replacement designs by 100 to 1,000 percent.

The switches produced in the 1900s share some or all of the following features:

• Tangential geometry (no entry kink angle)
• Spiral geometry (variable switch curvature)
• Housed switch points
• AMS switch points and stock rails
• Guardrails in front of the switch
• Guardrails in the switch

As stated earlier, the TAG selected the switch design for the project. Table 1 (pg. 70) lists the features of the SEPTA spiral switch turnout compared to an AREMA No. 8 lateral switch turnout. The objectives of the two switch designs are clearly different. The SEPTA switch turnout is built for ride quality, with smoothed transitions and spiral curves to minimize the rate of change of accelerations, and the AREMA switch turnout is built for economy, speed, and component durability.

By having a large kink (entry) angle and large radius closure curve, the design geometry of the AREMA switch turnout maximizes allowable speed under the “cant deficiency” rule — the Federal Railroad Administration’s rule for speed in curves. The SEPTA switch turnout designers were not subject to this rule, thus minimum switch closure curve radius and maximum cant deficiency were not important issues for them.

Figure 1 (pg. 67) compares the layouts of the SEPTA and the AREMA switches. The figure shows the geometry and alignment differences. The SEPTA switch spiral geometry produces a lower entry angle for facing point and a lower exit angle for trailing point movements.

Predicted Performance

TTCI modeled the performance of SEPTA B-IV cars operating over a SEPTA tangential spiral geometry No. 8 lateral switch turnout, an AREMA No. 8 lateral switch turnout, and several variations of these two designs. The study was designed to learn which switch design features contribute to the good performance and long service life of the SEPTA switches. These design elements can be applied to a modern switch design to develop a low-cost, high-performance switch for future use.