Beginners Ask Trains Ask MR: Flange-bearing frogs explained

Ask MR: Flange-bearing frogs explained

By Steven Otte | January 20, 2022

A 'lift frog' or flange-bearing frog lifts wheels on one route over the other

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A close-up image of a flange-bearing frog on a rail crossing diamond
Flange-bearing frogs explained: On a flange-bearing or lift frog, like this one on the diamond at Ridgley Tower, Springfield, Illinois, the main high-speed route crosses the diamond on a solid rail, while the secondary, low-speed route crosses over on its wheel flanges. Also called a “jump frog,” flange-bearing frogs are used on diamonds and turnouts where the secondary route is rarely used to reduce wheel wear on mainline trains. Bob Johnston photo

Q: What is a lift frog and why are they used? – Bob Kolankoski, Scranton, Pa.

A: Flange-bearing frogs, also known as lift frogs, jump frogs, or more colloquially leap frogs, are used on switches or diamonds where the main route sees far more traffic than the secondary. On a traditional tread-bearing frog, the flangeways – gaps in the rail – allow the routes to cross over each other, producing that “clickity-clack” sound as the treads hit the railhead again on the other side of the gap. This produces wear on the wheels as well as the frog. The idea behind the flange-bearing frog is to reduce that wear for trains on the main route and to let those trains cross the frog at full speed.

Here’s how it works. The mainline railhead through the frog is solid – no gaps, no clickity-clack, and no (well, much less) wheel wear. On the secondary route, though, the flangeway becomes a U-shaped channel that keeps the train on track as the wheels rise a few inches to bear on the flanges, not the tread. The wheels then pass over the railhead and flangeway of the main line, enter the channel on the other side, and are lowered to bear on the treads again.

Take a look at the photo above and you’ll see the wear on the main rail where the secondary crosses. To make such a precarious-sounding transit safe, guard rails keep the opposite wheels on the straight and narrow, and speed is restricted on the secondary route. This has given lift-frog crossings like the one in the photo the moniker of One-Way Low Speed diamonds, or OWLS.


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One thought on “Ask MR: Flange-bearing frogs explained

  1. I’m assuming then, that trains that cross on the secondary is running at a slow speed or slows down significantly when using this type of crossing?

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