Railroad Diamonds: At-Grade Track Crossings, Frogs & Interlockings

Last revised: September 2, 2026

A railroad diamond is not a switch. It is the piece of special trackwork that lets two lines occupy the same grade without joining. From the air it looks like a diamond: four frogs, four flangeway gaps, and a standing invitation for every wheel to pound steel. Railroads laid thousands of them because a flyover costs a fortune. Track gangs have spent the better part of a century living with the ones that remain.

The geometry is older than the diesel. Wherever two companies met and neither would climb over the other, they cut the rails and bolted a crossing together. Protection came later—interlocking towers, then remote plants, then a dispatcher two states away. The iron in the middle of the plant did not get easier. It got heavier, and the trains got longer.

Milwaukee Road U30B 6005 and Electro-Motive units lead a freight through the interlocking at Rondout Illinois June 14 1968

Milwaukee Road U30B #6005 and a trio of Electro-Motive units lead a short freight through the interlocking at Rondout, Illinois, June 14, 1968. The unit was five months old (GE serial 36500, January 1968); Milwaukee later renumbered the class into the 5600s. The diamonds here belong to the Elgin, Joliet & Eastern crossing of the Chicago–Milwaukee main. David Hawkins collection.

On this page: At a glance · What a diamond is · How a wheel gets across · Why the angle matters · Rigid, movable-point, and flange-bearing · Interlocking, not a switch · Why maintenance of way hates them · Rondout · Elmhurst · Where to see one

At a Glance

Item	Detail
What it is	At-grade crossing of two railroad tracks that do not connect. Named for the diamond shape seen from above
What it is not	A turnout (switch), a slip switch, a wye, or a highway-rail grade crossing. A diamond does not let a train change railroads
Hardware	Four frogs (one at each corner), guard rails, flangeways, and extra-long ties. A double-track crossing of a double-track line is four diamonds in a cluster
Frog vs. diamond	A frog is the crossing piece. A turnout has one. A diamond is an assembly of four
Common types	Rigid (fixed-point) manganese; movable-point (switch diamond); one-way low-speed (OWLS); full flange-bearing (FBF)
Protection	Interlocking. Historically a tower; today usually a remote plant or automatic interlocking. Conflicting routes are locked, not “switched”
Why they exist	Cheaper and faster than a flyover or underpass. Nineteenth-century networks crossed at grade because that is how the map filled in
Why they are loathed	Every axle slams a gap. Wear, batter, broken frogs, slow orders, and two railroads arguing over who pays

What a diamond is—and what it is not

Two running rails have to be continuous if a wheel is to stay on them. Where a second pair of rails crosses at the same elevation, continuity is impossible: the flange of a wheel on one line has to pass through the head of a rail on the other. The solution is a frog—a casting or fabricated piece with a flangeway cut through it—and a diamond is simply four of those frogs arranged so both routes can be used, one at a time.

That last clause is the whole operating problem. A diamond is a shared piece of real estate. One train at a time. The other waits. There is no diverging route, no interchange, no “taking the diamond” onto the other company unless a connecting track exists somewhere else in the plant. If you can change railroads at the crossing, you are looking at a junction with turnouts, not at the diamond itself.

A slip switch borrows diamond geometry and then adds switch points so a train can change tracks. It is a different, more expensive animal, common in tight terminals. A wye is three turnouts in a triangle. A highway crossing is a road, not another railroad. The glossary on this site files the frog with switches because that is where most people first meet one; the diamond is the same idea applied to a crossing instead of a turnout. See railroad property terms.

How a wheel gets across

On ordinary rail the tread of the wheel carries the load and the flange keeps the wheelset in gauge. At a conventional (tread-bearing) frog the running surface is interrupted by a flangeway. For a moment the wheel is unsupported, or barely supported, and then it lands on the other side. Guard rails opposite the frog hold the opposite wheel so the flange does not pick the wrong point and climb out.

On a turnout the frog is one event. On a diamond a car does it four times per truck if you count both rails of both tracks—twice per axle on the route being used, at two frogs in succession. High-angle (near 90-degree) frogs have a short gap; the wheel can jump it. Acute frogs stretch that gap. The longer the unsupported distance, the harder the landing, the faster the manganese batters, and the more likely the railroad is to hang a slow order on the plant.

Hadfield manganese steel—roughly 12 to 14 percent manganese—has been the standard frog metal for more than a century because it work-hardens under traffic. Rail-bound manganese (RBM) and solid manganese castings are the usual North American recipes. They still wear out. They still crack. A diamond is a consumable.

Why the angle matters

A 90-degree crossing is the tidy case. The flangeway gap is as short as the geometry allows, the frogs are compact, and the assembly is relatively short. Right-angle diamonds show up where two companies happened to meet square: urban grids, belt-line crossings, some Midwestern junctions.

Most of the map is not square. When two mains cross at 20 or 30 degrees the diamond becomes a long, skinny lozenge. The center frogs see a wheel that is almost running along the flangeway instead of jumping it. AREMA publishes a graph of when rigid center frogs are still acceptable; past that limit the industry reaches for movable points. The acute diamond is also a longer piece of special trackwork to surface, to keep in gauge, and to replace when it finally lets go.

A double-track railroad crossing another double-track railroad is not one diamond. It is four, laid as a cluster. Railfans call the whole plant “the diamonds.” Rochelle, Illinois, is the textbook: Union Pacific’s Geneva Subdivision across BNSF’s Aurora Subdivision, a four-diamond plant known on the railroad as NX Crossing.

Rigid, movable-point, and flange-bearing

Rigid diamonds are the historical default: four fixed frogs, open flangeways, guard rails, and a speed restriction. They are simple, relatively cheap, and hard on everything that touches them. Most of the diamonds in the photographs on this page are this type.

Movable-point diamonds (also called switch diamonds or movable-point crossings) close the flangeway on the route that is lined. A switch machine throws points the way a turnout does, so the selected route has a continuous running surface. The other route is blocked. Impact drops, speed can rise toward track speed, and the plant now depends on the interlocking as completely as a crossover. The hardware is expensive and it has to work in snow. Railroads spend the money where tonnage and delay justify it.

Flange-bearing frogs take the opposite tack: instead of jumping the gap, the wheel rides on its flange across a raised flangeway floor. The AAR’s test plant at Pueblo (now MxV Rail) spent the late 1990s and 2000s proving the idea under heavy axle loads. Two revenue-service patterns followed:

OWLS (one-way low-speed) diamonds keep the busy route tread-bearing and continuous. The light, slow route climbs onto its flanges and crosses over. Think of a lift frog in a turnout, applied to a crossing.
Full FBF diamonds are flange-bearing on both routes. BNSF put early examples in at places such as Shelby, Ohio, and Moorhead, Minnesota; later full-FBF plants include Lamar, Christopher, and Milano, Texas. FRA waivers covered the first installations because flange-bearing was new to the Track Safety Standards. MxV measurements show lower impact forces and fewer slow orders than conventional diamonds. The wear simply moves: flange-bearing bars, ramps, and guard rails instead of frog points.

None of this is “composite materials and embedded sensors” as a cure-all. Manganese, heat treatment, explosive hardening, and better ramps are the real catalog. Some plants now carry strain or temperature instrumentation. Most still get a track inspector with a gauge and a hammer.

Interlocking, not a switch

A diamond does not decide who goes first. The interlocking does. In the tower era an operator lined signals—and, where the plant had them, switches on connecting tracks—so that two routes could not be cleared at once. Mechanical interlocking (Saxby & Farmer and its American descendants) made it physically impossible to reverse a lever that would set up a collision. Power interlocking and, later, relay and solid-state plants did the same with circuits.

Quiet country diamonds often became automatic interlockings: first come, first served, with a timer so a second train does not follow a cleared route into a bind. Busy plants stayed manned into the 1990s and 2000s. Most of those towers are closed. The diamond is still there. A dispatcher in Omaha or Minneapolis now takes the place of the leverman who used to hoop up orders.

What a diamond does not have, unless it is a movable-point design, is a switch in the crossing itself. The frog is track. The plant is signals. Both have to work.

Why maintenance of way hates them

Every wheel on every train is an impact load. Frog points batter. Guard rails chew. Gauge spreads. Bolts work. In winter the flangeways pack with snow and ice; in summer the manganese flows. Two railroads share the bill, which means two engineering departments, two work windows, and a conference call about whose slow order is whose.

Alignment is unforgiving. A diamond that is a half-inch out of true will pick a wheel. FRA and railroad special-trackwork standards keep flangeway width, guard-check gauge, and frog condition on a short inspection interval. When the plant finally comes out, it is not a Sunday job with a spiker. It is a planned outage, a crane, and a prefabricated panel that has been sitting in the shop for six months.

The alternative is a flyover. Chicago’s CREATE program has replaced some of the worst city diamonds with grade separations—the Englewood flyover (2014) is the one most people can name—but a rural crossing of two secondary mains will keep its diamond until the traffic or the lawyers say otherwise. Cost still wins. The diamond is what you buy when you cannot buy the bridge.

Rondout: the plant in the hero photograph

Rondout, Illinois, is 32.5 miles north of Chicago Union Station on the old Milwaukee Road Chicago–Milwaukee main, today Metra’s Milwaukee District North with CPKC freight rights. In 1889 the Waukegan & Southwestern—construction affiliate of the Elgin, Joliet & Eastern—crossed the St. Paul here on its way to Waukegan. The two roads put up a tower southeast of the crossing. A brick-and-wood tower dating from about 1916–17 replaced the first one. Armstrong levers lasted until 1963, when Union Switch & Signal electric interlocking went in.

Just north of the diamonds the Milwaukee’s Libertyville / Fox Lake branch peels off; Metra still uses it. From 1902 until 1963 the Chicago North Shore & Milwaukee’s Mundelein branch flew over both companies on a fill. The J once had a second track through the plant; those extra diamonds came out on September 14, 1986. Canadian National bought the EJ&E in 2009. The tower closed at the end of second trick on March 7, 2015. Control went to the CP dispatcher. The structure still stands. Trains still bang the diamonds, the J usually at a crawl—period photographs of EJ&E SD38-2s here are often captioned at about 25 mph.

Rondout is also where the Milwaukee’s 1924 mail-train robbery took place, a mile and a half from the plant, but that is a different story. The railroad story is a belt line crossing a passenger-and-freight main, a tower that outlasted almost every other Milwaukee interlocking in Illinois, and a diamond that never did get a flyover.

Elmhurst: three railroads, then one

Chicago Great Western at left and Chicago and North Western at right diamond at Elmhurst Illinois July 1962 abandoned CA&E at far left

The Chicago Great Western (left) and Chicago & North Western (right) diamond at Elmhurst, Illinois, in July 1962. Far to the left, the abandoned Chicago, Aurora & Elgin. Today only the North Western remains, as Union Pacific’s Geneva Subdivision. Rick Burn photo.

Elmhurst stacked three companies in a few hundred feet. The Chicago & North Western main to the west—today UP’s Geneva Sub, still one of the busiest passenger-and-freight corridors out of Chicago—was crossed by the Chicago Great Western Chicago–Oelwein line. Alongside both, until July 3, 1957, ran the Chicago, Aurora & Elgin, the Roarin’ Elgin. CA&E freight lingered into June 1961. Rick Burn’s 1962 view already shows the interurban as a ghost on the left of the frame.

The Great Western itself lasted until 12:01 a.m. on July 1, 1968, when it disappeared into the North Western. C&NW wanted the Kansas City gateway, not a second Chicago approach. The Forest Park–Elmhurst CGW segment was abandoned in 1972; the Elmhurst depot and right-of-way went to the park district in 1971. The diamond in the photograph had perhaps a decade left. The C&NW iron is still there. The crossing is not.

That is the usual life cycle. A diamond is a treaty between two companies. When one of them dies, the treaty is cancelled and the special trackwork comes out. What remains is ordinary rail, and a photograph.

Where to see one

The plant that made diamonds a public spectacle is Rochelle, Illinois, about 80 miles west of Chicago. UP’s Overland Route (Geneva Sub, former C&NW) crosses BNSF’s Aurora Sub (former Burlington) on a cluster of diamonds at NX Crossing. The city opened a railfan park in the eastern quadrant on August 30, 1998—the first park in the country built for watching trains at a diamond. Tens of trains a day still take it at speed, with the usual pause when the other railroad has the plant.

Fostoria, Ohio; Joliet; Kansas City; Tower 55 at Fort Worth; and a dozen Chicago-area leftovers will also do. Stand at a legal public place. The diamond is railroad property. The sound you want is the one the photographs cannot give you: four frogs in a row, under a stack train, at whatever speed the slow order still allows.

Sources

AREMA Portfolio of Trackwork Plans: crossings, movable-point crossings, OWLS diamonds
MxV Rail / TTCI technology digests on flange-bearing frog crossing diamonds (1999 trials through TD22-023, 2022)
Federal Railroad Administration Track Safety Standards materials on FBF waivers (BNSF Shelby, Moorhead, Lamar, Milano, and related plants)
William C. Vantuono and industry coverage in Railway Track & Structures on FBF performance and next-generation designs
Pacific RailNews, July 1993, Rondout plant history; The Railroad Collection, Rondout tower closing (March 7, 2015)
Chicago Railfan interlocking notes; chicagorailfan.com CGW Chicago District abandonment dates (Forest Park–Elmhurst, 1972)
Railroad Picture Archives / RailPictures.net: Milwaukee 6005 built as U30B, GE serial 36500, January 1968; later 5605
City of Rochelle / Rochelle Railroad Park: NX Crossing, park opened August 30, 1998