Have you ever been to the railway station? Or look down and see what's under the rail when passing through the railway line?

A pile of dark wood was neatly stacked there.

A lot of people's first reaction was: was the wood smoked? Or kerosene?

To be honest, the answers are marginal, but not absolutely true.

Sleepers are black because they have undergone a corrosion prevention process called "vacuum high pressure oil immersion". The process goes without saying: soak the wood with an oil bath to let the preservative oil permeate from inside to outside. The wood turns black naturally after soaking.

The more uniform and darker the color, the better the bubble.

If you don't believe it, take a sleeper and cut it open. It is black on the outside and a certain depth in the inside. More than one centimeter from the skin to the inside, it is immersed in the anti-corrosive oil, which is true oil immersion.

Some sleepers on the market, looking black, but you're all open - good boy, the inside is still wood. It was brushed up, not the same as the oil.

How did the sleepers turn black?

Start with raw materials. Sleepers are mostly pine wood. Pine wood itself is light yellow, with straight texture and moderate texture. Sleepers are good materials. But it has a fatal weakness - fear of water, fear of insects, fear of decay.

Imagine, putting a pine directly under the rail, exposing it to the sun and rain, and within a few years it would be blistered and eroded by insects. How dare the railway use this kind of thing?

So the pine must "shower" first. And it's not a water bath, it's an oil bath.

Many people think that sleepers are simply thrown into the oil pool to make a bubble. That's simple.

Anti-corrosion treatment of railway sleepers is an industrial process with the core of "vacuum high pressure oil immersion".

The first step is pretreatment. Peel and cut the newly cut pine wood into sleeper blank with standard size. Then dry to reduce the moisture content to a reasonable range. This is a critical step - wood is too wet for preservative oil to enter; Too dry, wood may crack out before soaking.

The second step is vacuum pumping. Sleepers are placed in a huge sealed tank, which extracts air from the tank to form negative pressure. Why vacuum pumping? Because the air inside the wood is pumped away, a "cavity" is formed, making room for the preservative oil behind.

Third, high-pressure oil injection. Anticorrosive oil heated to the proper temperature is filled into the tank while high pressure is applied. Driven by the pressure difference between the inside and outside, the preservative oil is "forced" into the deep fibers of the wood. This process continues for several hours until the oil has penetrated into place.

Step 4: Maintain pressure and relieve pressure. Maintain high pressure for a period of time and then slowly release pressure to allow excess preservative oil to return.

Step 5: Curing out of the tank. The sleepers are taken out of the tank and the residual oil on the surface is naturally dried and solidified to form a protective film.

Upon the completion of the whole process, the penetration depth of anti-corrosive oil of sleepers can reach over 13mm. That is to say, even if the surface of the sleeper is rubbed off by the wind and sunshine for many years, the anti-corrosive coating inside is intact.

This is the fundamental difference between really oil-immersed sleepers and poor-quality painted sleepers.

It is also black, with different quality.

Qualified oil-immersed sleepers, anti-corrosion oil from the end to the side, the whole body is black, and the oil is bright. Cut so that the black inside extends more than 13mm inwards.

Poor quality painted sleepers with just one coat of preservative oil on the outside. It is black at first glance, but the inside is still wooden after cutting. This kind of sleeper cannot be used for several years. Once the surface oil layer falls off, the inside of the sleeper will be damaged by the rain.

How to distinguish? Give you three civil methods.

Looking at the section. Cut a section with a saw to see the section - paint the sleepers with only one coat of black on the skin and oil-immersed sleepers black from the outside to the inside, at least 13 mm deep.

Weight of secondary balance. Oil-immersed sleepers absorb enough preservative oil and are much heavier than plain sleepers of the same volume. Lifting a heavy hand indicates that the oil is immersed in place.

Three-drilling hollow hole. Generally, manufacturers have tools for detecting the immersion depth - hollow drills. Randomly select a hole to check the wood chips and measure the length of the black immersion part.

Sleepers are black like this?

In four words: durable.

When the preservative oil penetrates the wood fibers, it kills the fungus and eggs and stops the rotting fungus from growing. In wet and rainy railway, the service life of anti-corrosion sleepers can reach more than ten years or even decades.

The pine itself is elastic, and the wood fiber is more dense after anti-corrosion treatment, which can withstand repeated rolling of trains. One qualified corrosion resistant sleeper can bear dozens of tons of weight without any problem.

The ballast beneath the sleepers (i.e. the layer of crushed stone beneath the rail) and the sleepers themselves form a shock absorbing system. The fiber structure of pine is natural and elastic, which can effectively absorb the vibration and impact of train running and protect the subgrade from being damaged.

The black is not dirty, it is the result of a precision industrial process. Vacuum high pressure oil immersion gives a common pine the ability to withstand time, rain and heavy pressure.

One sleeper, one in black, carrying the safety of the entire railway.