Why are presumably new railroad tracks rust-brown?

Growing up, the only railroad tracks I ever saw were old tracks, because no new lines were being built, and I didn’t expect them to be any other color. After all, any time you see tracks in films or IRL, they’re always that same rusty reddish brown, right?

Flash forward to 1990 or so, when L.A. began building out its transit system. We’ve put in several new rail lines since then, and I now notice the same thing–brown tracks. So what I’d like to know is this: Are the rails purposely pre-oxidized before being laid down, because you need to do that for some reason? For that matter, come to think of it, just about all structural steel seems to be oxidized in the same way. A brand new bridge is hardly ever shiny, after all. That must be for a similar reason–so please tell me what it is?

Steel rusts very quickly, on the surface. I don’t know if it’s actually creating a passivating layer as some metals do, but for structural steel it happens rapidly on any untreated surface.

Between when they are rolled in the steel mill and when they are laid on the tracks, the rails may spend weeks or even months, mostly outside exposed to the weather. Naturally they rust.

Rust never sleeps.

Steel rusts, it’s never worth painting unless it’s being exposed to harsh conditions or if you want it a different color for aesthetic reasons.

If you watch a building being put up with brand new steel generally anything below the floor they are currently working on has rusted brown.

Yep. I’ve had exhaust pipes lying on the floor get sprayed with water; practically the instant the water dried, a reddish patina was left in its place. Unprotected iron and steel rusts almost instantly.

you might see the rust abraded off where it rubs. on rails and wheels it might be edges or only parts.

auto disc brakes can build up thin rust on a nonarid environment in a day.

Slight tangent: does the rust affect the structural integrity of the steel?

As long as the rails were manufactured well, only the surface should rust. The rust then forms a protective layer which keeps the rest of the steel intact.

Frequently used tracks will have any rust scraped off of them by the train wheels and will usually be fairly shiny. They do wear down after a while and have to be replaced.

Around here, whenever a section of subway or commuter rail track is shut down for repairs, it is generally completely rusted within two or three days.

Or years. There’s been piles of them along UP’s route here for almost a decade.

Not significantly.

The rust is a microscopically thin layer on the outside of a rail that is solid steel several inches thick. No doubt there is some degradation, but it would be almost immeasurable. The degradation from the wear-and-tear of heavily loaded freight trains would be much more.

Rust can indicate if a track is inactive. The trains on active tracks keep the tops of the rails almost rust free.

That knowledge comes in handy if you’re out in the country and walking down a train track. We walk down an inactive track every year getting to our deer camp.

It doesn’t affect the structural characteristics of the steel over any reasonable time frame–the type of steel we are talking about here is not sheet metal. Over long periods of time, and sooner if exposed to harsh conditions (especially salt water), then it will eventually affect the structural characteristics.

Also, when constructing a steel frame for a building, once the building is constructed, the steel is then protected from the weather, so the rate of corrosion decreases greatly. In structures that will be continuously exposed to weather (like bridges), the steel will be sandblasted to remove loose rust, and then painted or otherwise protected.

Tripolar and friedo, if the rust is reddish, it’s not the form of iron oxide that creates a passivating layer. The type of iron oxide that forms a tightly adherent passivating layer is a black oxide, like you see on a cast iron pan. In the case of rails, there is no coating (especially on the top surface) that can be applied that won’t be removed by train traffic, so they simply use a more corrosion-resistant type of steel, along with a thick cross section of steel. When the steel corrodes beyond allowed limits, they then simply replace the rail.

As a benchmark, marine experts work on 0.1mm (.004 inch) per year of corrosion loss of unprotected steel in a salty and oxygen rich environment.

I believe that depends on the formulation of the steel - there are grades of steel that do form a reddish, very-rust-like passivating layer - for example, COR-TEN (famously used for public art such as The Angel Of The North.

The black on a seasoned cast iron pan isn’t iron oxide (well, certainly not entirely) - it’s a layer of carbonised/polymerised oil.

This conforms to my observations. When the black oxide sets in, the corrosion slows down considerably. Pitting still happens, but the formation of ‘scale’, the loose red stuff is cut down a lot. I suspect the red oxide is water soluble because the formation of that can continue at a rapid pace until it consumes all the steel.

ETA: And an untreated cast iron pan will rust.

I was always under the impression that it was mostly black iron oxide–but am not finding much online to support that contention–so I may well be incorrect in that assertion.

Perhaps blueing steel would be a better common example of the formation of a passivating layer of black iron oxide.