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Flange-bearing frog

A one-way, low-speed (OWLS) diamond in Champaign, Illinois. This is an example of flange-bearing frogs in use on North American freight rail lines.

A flange-bearing frog, often abbreviated FBF, is a type of frog in which the flange of the wheel on a railway vehicle supports the weight of the vehicle. In conventional practice, the tread of the wheel rests on the head of the rail and bears the weight of the vehicle, while the flange is used to keep the vehicle in the gauge of the track. Flange-bearing frogs are a relatively recent development, as a means to reduce maintenance costs associated with turnouts and diamonds, where rails must cross one another.


Flange-bearing frog (bottom, center) used in street railway trackage in Liberec, Czech Republic. Note the polishing in the flangeway indicating contact with the wheel flange.

Flange-bearing frogs have been used by street railways for more than 200 years as a means of reducing noise pollution in populated urban settings.[1] In 1995, Robert Willow filed a patent in the United States adapting this concept for use in mainline freight and passenger service. The patent describes the technology as, "designed to support a railroad wheel to roll through the frog on its flange rather than requiring its tread to jump across a flangeway gap." Ramps are placed on either side of the frog to gradually transition the wheel load from the tread (known as tread-bearing loading mode) to the flange (known as flange-bearing loading mode) and to lift the wheel so that the tread is completely clear of the rail.[2]

Advantages over conventional frogsEdit

Conventional frogs require a gap of almost 3 inches (76 mm) in the running surface of the rail to allow adequate clearance for the flange of a railroad wheel. When a wheel in tread-bearing mode encounters this gap at high speed, it generates high dynamic loading (particularly impact loading) on the edge of the rail at the gap.[1][3] The repeated application of this impact leads to significant metal fatigue and eventual failure of the frog components, which translate directly to increased maintenance costs, train delays to allow for repairs, and an overall reduced lifespan for the frog and track structure.[4]

Because the flange rides on a continuous surface in a fully flange-bearing mode, the impact loads generated by the flangeway gap of conventional frogs are eliminated. This reduces maintenance and extends the life of the frog, which leads to an operational cost savings that exceeds the increase in initial costs, when compared to conventional frogs.[1] As previously mentioned, flange-bearing frogs are quieter than conventional frogs—again, because of the reduced impact loads—which provides a potential benefit when implemented near residential areas.


Several types of flange-bearing frogs have been developed for specific applications on North American railroads. These frogs may be flange-bearing in one or both legs of the frog, and may by used either as part of a turnout or as part of a diamond.

Lift or jump frogEdit

Lift frogs are used in turnouts where the through (straight) route is heavily used, and the diverging (curved) route is only used on occasion and at low speed. The frog is tread-bearing, with a normal rail surface, on the through route, and is flange-bearing on the diverging route. The name is derived from the wheel on the diverging rail being lifted over the through rail. No guard rail is required on the through route, but is critical on the diverging route to keep rail vehicles in gauge. Common applications are set-out tracks or lightly used industry tracks (both of which are special types of sidings diverging from high-speed, high-tonnage mainlines.[4])

Combination tread- and flange-bearing frogEdit

Combination tread- and flange-bearing frogs are used in turnouts and are designed to account for various stages of wheel wear (in which the tread of the wheel will wear thin, effectively increasing the flange height):

  • When the frog is new, all wheels will cross the frog in a fully flange-bearing mode, with no contact between tread and frog.
  • As the frog is used, the flangeway will abrade against the wheel flanges and deepen. In this case, new wheels (those with shorter flanges) will make contact between tread and frog, creating a combination tread- and flange-bearing loading on the frog. Meanwhile, worn wheels (those with taller flanges) will continue to impart a fully flange-bearing loading onto the frog.
  • As the frog continues to be used, the flangeway will deepen further until new wheels will cross the frog in a fully tread-bearing mode, while worn wheels will cross the frog in a combination tread- and flange-bearing mode.

It is worth noting that as the tread-bearing loading environments begin to manifest themselves, the same impact loads which cause trouble with conventional frogs will return. They are almost exclusively found in rail yards, where rail vehicles operate at or below 10 miles per hour (16 km/h), due to the reduction of guidance keeping the vehicles in the gauge through the turnout.[4]

One-way low-speed diamond frogEdit

One-way low-speed diamond frogs are used in diamond crossings, often referred to as OWLS diamonds. These types of frogs are analogous to lift frogs in turnouts: the higher-trafficked line crosses the diamond on a normal rail surface in tread-bearing mode, and the lower-trafficked line crosses over the higher-trafficked line in flange-bearing mode. Because there is no flangeway gap to cross on the higher-trafficked line, vehicles using this line can cross the diamonds at the maximum speed allowed by the track design. Because the lower-trafficked line is in a situation where gauge restraint is reduced and because it has to cross over the flangeway gap for the higher-trafficked line, vehicles using this line are limited to speeds at or below 10 miles per hour (16 km/h). OWLS diamonds are commonly used where a rail line with very little traffic operating at low speed crosses a rail line with considerably more traffic operating at higher speeds.[4][5]

Full flange-bearing diamond frogEdit

Full flange-bearing diamond frogs are also used in diamond crossings. These frogs are flange-bearing for both lines through the diamond. Because both lines are flange-bearing, there is no need to elevate the flanges of vehicles using one line, as there is in the case of OWLS diamond frogs, which allows for greater gauge restraint. Consequently, vehicles can cross these frogs at the maximum speed allowed by the track design; regardless of which line on which they cross the diamond. Though there are very few of these in use today, full flange-bearing diamonds can be found where two high-speed or high-tonnage main lines must cross one another at-grade.[4][5]


In an effort to regulate the depth of flangeways in a crossing frog, the Federal Railroad Administration created rules which, by definition, prohibited the use of flange-bearing frogs on track with speed limits greater than 10 miles per hour (16 km/h).[6] Because of this, an FRA waiver is required to install full flange-bearing diamonds. Subsequent to installation, an extensive battery of inspections of both the diamond itself, and rail vehicles which cross the diamond, to ensure that safety is not compromised by the installation of such a diamond.[7] This may change in the future, as FBF becomes a proven technology, but currently the rigorous inspections are a deterrent for widespread installation of full flange-bearing diamonds.[5]


  • As of Spring 2010, BNSF Railway had installed more than 100 lift frogs across their system, with plans to install many more.[4]
  • In September 2009, BNSF installed three combination tread and flange-bearing frogs in their Lafayette, Louisiana yard. Since installation, the frogs have worn as described above, and there is evidence that some wheels already cross the frog in a combination tread- and flange-bearing mode.[4]
  • As of Spring 2010, BNSF had installed 22 OWLS diamonds on its system; as of December 2008, CSX had installed 13.[4][5]
  • In 2006, CSX installed a full flange-bearing diamond at Shelby, Ohio, the first application of this technology on a North American freight railroad. In 2008, BNSF Railway installed two such diamonds at Moorhead, Minnesota. Both installations are currently under FRA-mandated inspection, as described above.[4][5]


Though tests are still ongoing, results in both a controlled environment and in the field are promising. The Transportation Technology Center in Pueblo, Colorado has installed an OWLS diamond on their property, where they perform research for various railroad entities. Their results show that dynamic loads from a train crossing an OWLS diamond are half the loads from the same train crossing a conventional diamond at half the speed it crossed the OWLS diamond.[8] The inspections of both track and vehicles by BNSF and CSX at their full flange-bearing diamonds have shown no unexpected wear, and suggest that such diamonds could be implemented on a larger scale.[4][5]

See alsoEdit


  1. ^ a b c Barkan, Dr. Christopher. CEE 408 - Railway Transportation Engineering, Class Notes. University of Illinois at Urbana-Champaign, 2011. Print.
  2. ^ Willow, R. (1996). U.S. Patent No. 5,531,409. Washington, DC: U.S.
  3. ^ Judge, Tom. "A flange-bearing frog concept for heavy rail freight operations - rail crossings track - Vehicle Track Systems Newsletter" Railway Age. June 1995. Print, web.
  4. ^ a b c d e f g h i j Armstrong, M. (2010, May 14). Overview of Flange-Bearing Frogs. Presentation given at semiweekly William W. Hay Railroad Engineering Seminar, University of Illinois, Urbana, IL.
  5. ^ a b c d e f Clark, D., O'Connor, T., and Davis, D. (2008). Update on the Use of Flange-Bearing Technology in Special Trackwork. Annual Conference of American Railway Engineers and Maintenance-of-Way Association. Retrieved November 17, 2011, from
  6. ^ Track Safety Standards Compliance Manual. Federal Railroad Administration, 2009. Print, Web. Track Safety Standards Compliance Manual Archived 2009-07-02 at the Wayback Machine
  7. ^ Cerny, Louis. "New Federal Railroad Administration waiver allows unlimited number of flange-bearing frog crossing diamonds to be installed." Archived 2012-04-19 at the Wayback Machine Railway Track and Structures. January 12, 2011. Print, web.
  8. ^ Davis, D. (2009). Low-Impact Special Trackwork Research at Transportation Technology Center, Inc.. Presentation given at the annual conference of the Transportation Research Board, Washington, DC.

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