Why do trains screech

Noisemaker

The essentials in brief...

The train is an environmentally friendly, but noisy means of transport. At the end of 2015, over 80,000 people across Switzerland were still exposed to rail noise pollution that was above the limit values.
The railroad: environmentally friendly - but loud

The composition of the noise of a train depends on its speed. At normal travel speeds, the rolling of the wheels on the tracks accounts for the largest share of the rail noise.
Rail noise is a mix of sounds

The design of the brakes has a major influence on noise emissions. While loud block brakes were used in the past, new trains are now only equipped with noticeably quieter disc brakes.
The old brakes make trains loud

The noise emissions from block brakes are reduced by up to 10 dB if gray cast iron brake blocks are replaced by ones with a higher proportion of plastic.
What can be done about rail noise?

In the case of tram noise, which is also part of rail noise, the squeaking of curves is the loudest and most unpleasant noise. The use of rail lubricant can minimize squeaking.
The squeaking of the trams

If trains regularly pass a reception point, their noise is not perceived as just as annoying as street noise that is equally loud. This was taken into account in the LSV with a bonus, but is questionable today due to the high utilization of the rail network.
The rail bonus

The railroad: environmentally friendly - but loud

The Swiss rail network is the most heavily used in all of Europe. The train is an efficient and environmentally friendly means of transport, but it is often an annoying source of noise for people who live and work near train lines. Residents of goods transport routes in particular are affected by high levels of noise pollution: According to the Swiss Federal Railways (SBB), 230,000 people in Switzerland were exposed to noise pollution from railways in 2000 that was above the limit values. Fortunately, after extensive renovation measures on the part of the SBB, the number was reduced to the still beatific number of 83,000 people exposed to noise at the end of 2015. More people are affected in the densely populated central plateau than in rural areas.

Prof. Markus Hecht, head of the rail vehicle department at the Institute for Land and Sea Transport at the Technical University of Berlin and leading expert on rail noise, said in an interview with the portal of the rail industry eurailpress.de in October 2015: "At the moment we are ( The railway, editor's note) is the worst mode of transport in terms of noise pollution. That is not possible if we claim to be the most environmentally friendly at the same time. " The full interview can be found at the end of the chapter.

Source: sonBASE noise database, Federal Office for the Environment (FOEN).

The sonBASE noise database allows precise mapping of rail noise exposure.
Example: Noise pollution from rail traffic around Zurich's main train station during the day.

The GIS-based noise database sonBASE of the Federal Office for the Environment (FOEN) enables scientifically sound comprehensive evaluations of noise pollution in Switzerland. For the calculation of the rail noise, model calculations of over 3,000 km of railway lines were taken into account. sonBASE maps the noise pollution from rail traffic during the day and night.

Interview with rail noise expert Prof. Dr. Markus Hecht (TU Berlin)
To the GIS noise database sonBASE

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Rail noise is a mix of sounds

The overall noise of rail traffic essentially consists of three components:

Drive noiseRolling noiseAerodynamic sound

Other sources of noise are ventilation systems, braking and squeaking in curves.

At speeds between 50 and 270 kilometers per hour, the main cause of rail noise is the rolling of the wheels on the tracks. The rolling noise is transmitted as structure-borne noise via the wheels and axles to the bogies and other parts of the train and passed on within the rail. The structure-borne sound is transmitted to the air and is therefore audible.

At speeds below 50 kilometers per hour, for example in train stations, the engine noise from the locomotive is dominant. The level of drive noise is influenced by the drive motor, gearbox, fan, air conditioning, compressor and wheel flange lubrication elements. Anti-vibration noises are transmitted into the vehicle interior and outside as airborne and structure-borne noise. Brake noises are also attributed to the drive system.

Aerodynamic noises only come into play at speeds of over 300 kilometers per hour. These are mainly caused by flow and vortex shedding of the pantograph and can hardly be shielded with noise barriers.

The drive noise is only slightly dependent on the train speed. On the other hand, the rolling noise increases by the third to fourth, and the aerodynamic noise even by the sixth power of the driving speed.

As soon as these noises are perceived as annoying, they are classified as noise which, due to its structure and volume, is considered to be stressful or harmful to health for people and the environment.

Dosto 20 dB quieter than EW I / II

The noise emissions are heavily dependent on the construction of the rolling stock. At SBB, for example, the intercity double-decker coach (Dosto), which was still in frequent use in 2017, is 20 dB quieter than the standard coach (EW I / II), which, however, is rarely sent on the rails. This roughly corresponds to the difference between solid chamber music and a groovy rock concert.


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The old block brakes make trains loud

About 25 years ago, scientists discovered that, after a while, undulating irregularities were imprinted on the rail profile and the wheel tread. They called these bumps "ripples". The main reason for the corrugation is the vibration behavior of the track. The track is excited to vibrate by passing trains, depending on the frequency. If it does not resonate, very high forces act between the wheel and the rail, which after a while causes the regular bumps.

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Corrugated and ground rails in comparison.

The corrugations are amazingly even: the distance from Riffelberg to Riffelberg is three to eight centimeters. The difference in height between mountains and valleys is usually less than a tenth of a millimeter, which you can just feel with your fingers. Nevertheless, the corrugations lead to higher noise emissions. In addition, there is extreme stress on the wheel and track - and thus increased wear.

The type of brakes has a decisive influence on the formation of corrugations: In the past, gray cast iron block brakes were used, which press directly on the wheel surface and generate a noise level of 92 dB at a speed of 80 km / h at a distance of 7.5 meters. This type of brake increases the corrugation because it roughen the wheel treads. According to the FOEN, noise emissions can be reduced by 9 dB (max. 83 dB) even with plastic brake blocks instead of gray cast iron.

Noise emission of various brakes compared to disc brakes (driving speed 80 km / h).

Source: www.isi.ee.ethz.ch

Material and type of brake, comparison of the wheel treads. Gray cast iron block brake (left), plastic block brake (center), disc brake (right).

With the new generation of disc brakes (as of 2016), which do not press on the wheel tread but use their own braking surface, the noise emissions can even be reduced by 14 - 17 dB compared to the old gray cast iron block brakes. At SBB, new rolling stock is only equipped with disc brakes in the passenger transport sector. The older models of the fleet are still equipped with block brakes, the refurbishment of gray cast iron brake blocks to plastic brake blocks was completed in 2014.

Source: www.isi.ee.ethz.ch

Schematic representation of a disc brake.

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Block brakes of a freight wagon in a large and detailed view.


Freight traffic causes the most noise

In Switzerland and especially in the rest of Europe, freight traffic is responsible for the lion's share of noise emissions, as the number of old freight wagons with gray cast iron brake blocks is still high. In this country, the SBB's fleet of freight wagons (6267 wagons) had already been converted from noisy gray cast iron to quieter plastic brake blocks at the end of 2010. Plastic brake blocks can be divided into K and LL blocks, with LL blocks being used to convert existing old freight wagons with gray cast iron brake blocks. K-blocks are installed on new freight wagons. In Switzerland, the first tests are already running with the even quieter "freight wagon of the future", which is equipped with disc brakes. In March 2017, the FOEN also launched an invitation to tender for CHF 4 million, the aim of which is to make freight wagons even quieter (no more than 79 dB). In Germany, on the other hand, the owners of rolling stock have until 2020 to convert their wagons. In other European countries, efforts to reduce noise in rail traffic are even less advanced.

The pan-European goal of noise protection is a comprehensive renovation of the rolling stock: If only a few wagons of a long freight train have not been converted, the noise-reducing effect is barely noticeable due to the logarithmic nature of the decibel scale. In the Frankfurter Allgemeine Zeitung, Prof. Markus Hecht formulates three rules for adding sound to illustrate: Loud and loud is loud. When two equally loud freight trains meet, it sounds almost as loud as when only one such train passes by. Because our ear can only distinguish the resulting sound difference of three decibels with difficulty. Further: Loud and quiet stays loud. If only half of the freight cars in a train are equipped with quiet plastic brake blocks, but the other half still have loud cast iron brakes, then the train is almost as loud as it used to be. Finally, the third rule is: Only quiet and quiet results in quiet. A significant noise reduction can only be achieved with complete retrofitting of all freight wagons crossing Switzerland.

Source: SBB Cargo

From 2020, only freight wagons with quiet plastic brake blocks or disc brakes will be allowed to drive through Switzerland.

To achieve this goal, Switzerland has issued a ban on noisy freight wagons with gray cast iron brake blocks from 2020. DETEC estimates that it will take another 30-40 years from 2013 until all loud freight wagons across Europe are converted to quiet plastic brake blocks.

SBB Cargo: "The freight wagon of the future" (Blog, 2017)
Sda / Blick: SBB presents the "5L train"

The roar of empty freight cars

Structure-borne noise generated in the bogie of railway wagons can be transferred to the car body and cause a roar there. This is especially the case with empty freight wagons, where the wagon structure amplifies the structure-borne noise and converts it into airborne noise on the large surfaces of the wagon walls. At high speeds, the aerodynamic noise caused by vortex formation in the area of ​​the bogies is also important. A cladding of the bogie could serve as a measure to reduce this noise.

Bogie of a double-decker car.


Ballast bed as a sound absorber

The nature of the railway line influences the reflection of the sound: a ballast bed absorbs the sound more than a solid roadway. That is why slab tracks are only used in tunnels in Switzerland.

The tunnel bang

When a fast train enters the tunnel, a pressure wave is generated that leads the train at the speed of sound. At the other end of the tunnel, part of the pressure wave emerges from the tunnel mouth and, under certain conditions, can lead to a loud bang. The tunnel bang can be avoided by using suitable tunnel constructions.

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What can be done about rail noise?

Basically, smooth wheels on smooth rails are the best protection against unnecessary noise emissions. As described above, this can be ensured on the one hand by replacing the gray cast iron brake blocks with more modern and quieter plastic or disc brakes. On the other hand, well-ground rails primarily reduce high-frequency noise emissions.

Comparison of grooved and sanded tracks. In the high frequency ranges, the emission level can be reduced by up to 8 dB through well-ground tracks.

Only when the measures described above have been taken do additional measures, such as cladding the bogie, make sense, but this is technically demanding and complex. Furthermore, wheel and rail absorbers, which absorb the sound waves directly at the source, could possibly be used more frequently in the future.

Source: www.inct.de/d-index.htm

Rail absorbers reduce noise emissions.

In anticipation of the NEAT, the Einstein program on Swiss television SRF devoted an article in 2010 to the subject of "Noise from freight traffic and what is being done about it": SRF Einstein Noise measurements next to the rails.

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The squeaking of the trams

In the case of the tram, rolling noise dominates the noise level on straight stretches even at speeds of over 30 kilometers per hour. In curves with small radii, screeching and squeaking noises occur as the loudest and most unpleasant partial noises.

The forced guidance of the wheels when driving through curved tracks with small radii means that the wheel flanges touch the rails at periodic intervals, in contrast to straight travel, which results in the squeaking of curves. In addition, the rails are worn and uneven as a result, which can lead to an increase in the emission level of up to 15 dB (A).

Measures against tram noise

By regularly processing the wheels, covering the side of the wheels and lining the floor of the car with sound-absorbing material, the noise emissions from the rolling stock can be minimized. Regular grinding of the rail running surfaces prevents the formation of corrugations and thus prevents an increase in the sound level. Sound-absorbing routes made of gravel, lawn or absorption plates on the solid roadway reduce reflections and thus noise emissions.

Source: www.srf.ch

For decades, André Kofmehl has been working on a solution that can reduce the annoying squeaking of the rails to a minimum. Now the Swiss has found a solution that should even completely eliminate the squeak.

In Zurich, for example, rail lubrication systems are installed in almost all curves to prevent squeaking noises, through which lubricant can be applied to the roadway if necessary. These electronic lubrication systems specifically reduce the squeaking of curves at critical points. The approaching trams send out impulses and start the lubrication system. This presses the lubricant through tiny drill holes onto the rail and prevents it from squeaking in curves. Another positive feature of the lubricant is that it prevents the trams from slipping when braking. Measurements indicate that this can reduce wear on the rails by 10 to 20 percent.

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The rail bonus

The high noise emissions from rail operations suggest that the disruption from the railways is greater than that from road traffic. But this is refuted by various sociological studies. The emission of unpleasant high and low frequencies is lower with passing trains than with road traffic. In addition, the regular nature of rail noise and the acceptance of the general benefit of rail transport make people perceive rail transport more quietly.

The lesser disruptive effect of railway noise compared to road noise is taken into account in the Noise Protection Ordinance (LSV) by reducing the noise actually emitted by a minimum of 5 to a maximum of 15 dB. The level correction is calculated using the following formula:

K1 = -15for N <7.9
K1 = 10 log (N / 250for 7.9 <= N <= 79
K1 = -5for N> 79
K1 = Level correction [dB]
N = Total number of train journeys per day, resp. at night [-]
 = Number Train journeys per hour * no. Day / night hours
Night hours from 10 p.m. to 6 a.m. (8 hours)
Day hours from 6:00 a.m. to 10:00 p.m. (16 hours)

If, for example, 100 train passages are registered on a route per day, 5 dB are subtracted from the measured sound level. Even with 400 train passes per day (this corresponds to one pass every 2.4 minutes), the noise level is reduced by 5 dB. The level correction is also made when the train densities are so high that they no longer appear regularly to residents, but appear as random as road traffic. Under these circumstances, a level reduction is not justified. In the case of train routes with very high traffic density, it would therefore make sense to adjust the LSV, as the rail network is now much more heavily used than when the rail bonus came into force.

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