THE crash of wagons and shriek of braking shunters working at hump yards are an inevitable inconvenience to the sensitive ears of city dwellers.
Traditionally hump yards were located away from residential locations. But an increase in suburban swell in the past 20-30 years means that more and more people are living closer to these important freight terminals where noise often exceeds 130dB.
There are several causes of hump yard noise. Brake noise is one and this varies from the sounds emitted by incoming trains, shunting locomotives and brake testing, to one of the loudest sources, braking of wagons by hump retarders. Rolling noise from wagons is another major source.
The European Commission considers railway noise as a major environmental problem stating in a paper from 2005 that "there is high potential for reduction of railway noise in Europe" and subsequently enacting legislation to encourage railways to reduce noise emissions.
Barriers have been widely used as a means of reducing noise, including at hump yards where reductions of 3-7dB have been recorded, depending on the design and height of the barrier.
Studies have shown that constructing an environmental sound barrier can improve sleeping conditions for residents. However, in many instances following the installation of barriers residents have complained about a loss of sunlight and restriction of views and access to their home, reduced air circulation as well as poor maintenance of barriers. Maffei's 2013 study into the impact of the barriers, published in the Science of the Total Environment journal, found that people living behind a noise barrier quickly forget the previous noise levels and became more dissatisfied with the loss of a view.
Transparent barriers are now available that retain views but the arguments against their installation have resulted in an effort to reduce noise levels directly from the source.
This is consistent with schemes already underway to reduce rolling stock noise. For instance the UIC's recently concluded Eurotrain project is investigating the use of LL-block brakes, or silent brakes, which while replacing the brake blocks avoid a complete replacement of a wagon's braking system. The UIC hopes that the success of the trial will result in a programme to retrofit Europe's entire fleet of 600,000 wagons with the low-noise brakes.
While these measures will improve conventional braking, it will not affect noise at hump yards. Most hump yards use retarders to regulate the speed of wagons after they leave the hump. However, the high-pitch squeal often emitted by these devices has encouraged some manufacturers and operators to explore alternatives.
The source of this noise is the contact between the steel brake beams of the retarder and the wheels as they pass. The retarder manufacturing business of Sona, Germany, which was recently acquired by Siemens, has incorporated a noise abatement solution into its Beam Retarder TW-F for two rails and TW-E for single rail solutions which have been trialled at German Rail's (DB) hump yard in Nuremberg.
Sona's system uses a cast composite metal developed by Micke Brühmann, Germany, a manufacturer of sinter metal friction materials. These are bolted to the brake beam on the system's five force application units with a damping plate used to separate these segments from the brake beam. Trials conducted in Nuremberg have shown that the system reduces friction noise by up to 20dB with replacement and reconditioning of individual pads scheduled for between 5 and 7 million axle passes.
A radically different solution developed specifically for hump yards by Elpa, Slovenia, is the anti-noise system Bremex Annsys Basic which tests have shown is capable of eliminating high frequency braking noise of 130dB by 99%.
The system is suitable for all types of brakes and is applied adjacent to the track before the wagon passes through the retarder. The system works by applying an environmentally-friendly composite material to the wagon wheel flank in contact with the rail brake as the wagon passes the applicator. Applying the material directly onto the wheel creates an intermediate layer of material which is thermally decomposed during the braking process but without impacting the properties of braking. During the process the released kinetic energy is converted into heat and not sound, thereby reducing the braking noise at its source.
Figure 1 represents noise events measured before and after installation of the system. However, because other noises are present which reach 90dB or more, these noises limit the measured common effect of this system by up to 30dB.
The system is also in use at DB's hump yard in Nuremberg, where the results in testing phase between 2009 and 2011 led to a full rollout of the system.