Switch Gap Design

by Ken Scheer, from Modeltec, July 1993, reprinted by permission of author

Thankfully, the International Brotherhood of Live Steamers has set Wheel and Track Gauge Standards which are generally adhered to by most people building in The Hobby. This permits one person's equipment to be operated on someone else's trackage of the same gauge, be it a club or otherwise (after noting minimum curve radius compatibility), and to do so without fear of damage to either the equipment or the trackage due to constructional differences. A point of some concern, however, is that no track turnout (switch) guardrail flangeway gap standards have been set by the IBLS. After some study, it appears this task would be difficult to achieve at best, for reasons that will become apparent shortly. This article will attempt to try to clarify some of the design problems encountered in switch guardrail and frog construction in order to show how you go about finding the minimum and maximum allowable flangeway gap tolerances. We will also look at some other limiting design factors for track switches used on any track gauge from 3-1/2 inch to 7-1/4 or 7-1/2 inch. Though the IBLS does offer suggestions for recommended flangeway gap dimensions, these may not always be applicable for all cases due to subtle differences occurring in switch construction.