Switches for Good Performance and Long Service Life
A TCRP project was conducted to harvest the wisdom of the early transit switch designs and to retain the features that contribute to good performance in current and future designs.
No discussion of switch geometry is complete without including the rest of the turnout.
The selection of switch geometry affects the turnout closure curve alignment. The relative lengths of the SEPTA car versus the switch and turnout make inclusion of the entire turnout essential to the study. The switch is 13 feet long, with the lead length of the No. 8 turnout being 57 feet. The B-IV car is 68 feet long, with the truck spacing producing a 48-foot wheelbase. Thus, when the trailing truck of the car is moving through the switch, the lead truck is moving through the frog. Thus, the track modeled included the entire turnout with track beyond the frog being tangent at the frog angle.
To reflect the actual analysis done, the term “turnout” is used here, even though the focus of the study is clearly on the switch design. In most scenarios modeled, the switch produced the maximum forces or accelerations. However, in some operating scenarios, the turnout closure curve alignment may generate the maximum forces or accelerations.
The as-built SEPTA No. 8 tangential, spiral geometry switch turnout performs well in comparison to the per-plan AREMA No. 8 secant, circular geometry switch turnout. In its intended service of 5- to 15-miles per hour operation, the SEPTA switch is superior to the AREMA switch in minimizing loads and accelerations. However, the SEPTA switch turnout does show higher than desired accelerations under the 68-foot-long B-IV car operations. This may cause ride quality concerns at higher speeds.
Parametric studies of some design features were conducted to determine their effects on switch performance. Tangential switch entry is essential to the good performance of the SEPTA switch. Elimination of a kink angle and its resultant abrupt spike in dynamic loading produce a smooth ride and more even switch wear. The spiral switch entry and exit curves are effective at smoothing the ride through the switch by minimizing jerk or evening out the change in the accelerations. However, the very small radius of the closure curve may adversely affect the ride quality of certain types of equipment (i.e., longer cars) or service (higher speeds).
Figure 2 (pg. 71) shows the relationship of maximum lateral wheel force to speed and switch design for No. 8 (~59-foot lead length) turnouts. The SEPTA switch turnout is labeled Tangential Spiral. The AREMA design is labeled Secant Circular. A variant of the AREMA design with tangential alignment and a smaller closure curve is labeled Tangential Circular.
In this particular case, the advantages of the SEPTA switch turnout are not fully realized because of the longer length of the B-IV cars. These cars are longer than the turnout (as well as the cars operated when the switch was developed.) The extremely short radius closure curves of the turnout cause the cars to “stringline,” creatingrelatively high lateral forces. These forces increase rapidly with speed. The AREMA designs, with their larger radius closure curves, better accommodate the longer cars at higher speeds.
The use of AMS castings for the switch points in the SEPTA switch produces a tough and durable switch point. When introduced, the AMS point was vastly superior to rail steels of the time. However, modern rail steels perform as well as AMS in curve wear. The layout of the switch, with good geometry and guardrails, make the advantages of AMS almost redundant. The high cost of fabricating AMS switch points makes it an uneconomic choice for modern switches.
The housed switch point is a feature that provides benefits for switches with significant diverging traffic. The switch point is thickened to make it more robust and to diminish the risk of split switch derailments (Figure 3 (pg. 72)). The stock rail is diminished to accomplish this, which may result in a foreshortened life for this component. Housing would help to eliminate the sharp dynamic loading and localized wear at the point of switch on AREMA switches seen in the field, as well as those seen in NUCARS simulations done for the study. The running surface discontinuity at the point of switch seen at the gage face is especially important if guardrails are not used in the switch.
