Use of guardrails in the switch is needed for safety reasons. The guardrails ensure that a safe operation is maintained as the switch wears and deforms. The guardrail and back of wheel flange contact are nearly vertical, even on worn components. Thus, wheel climb is less likely than with worn wheel flange/switch point contact. As for switch performance, the dynamic loads in the switch are little changed. Switch point life is improved by transferring wear from switch point to guardrail.
The parametric studies identified design features that contribute to the good performance of early 1900s vintage transit switches. A comparison was made to an AREMA-style nontangential, circular curve alignment turnout. The study 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 AMS switch point, do not improve performance over modern rail steels. The particular circumstances of the line operations will affect the results of any such study. Key operating parameters include the desired operating speed, the percentage of diverging traffic and the length of the vehicles. MT
David Davis is a senior scientist with the Transportation Technology Center Inc.