Curve noise on the Bernese streetcar network
Anyone who has ever waited for their rendez-vous at the Zytglogge or drunk their morning espresso in one of the cafés on Theaterplatz in Bern knows the noise: the squealing of streetcars, which can lead to high noise pollution for a short time, especially in small curve radii - such as those found at the Zytglogge. Since streetcars operate primarily in conurbations, a large number of residents are potentially affected by the squeaking of streetcars. The motivation of the municipal transport companies to find efficient measures against these unpleasant noises is correspondingly high.
Video: Sound and image recordings of two crossing streetcars. During the first passing, a flange hiss can be heard, during the second passing, the curve screech dominates.
Search for efficient, maintenance-extensive measure
A proven measure to reduce the noise of curves on railroads is the lubrication of the tracks. On the urban streetcar network of BERNMOBIL, permanent lubrication systems are in operation at all neuralgic curve situations. This measure has significantly reduced curve noise in recent years. Due to the low flexibility and the high maintenance and renewal costs associated with the many systems on the curves, BERNMOBIL invested in recent years in the development of a new, mobile lubrication system, which are installed on the streetcars themselves.
Figure 1: The current system is based on stationary installations, which spray lubricant onto the streetcar rails from several nozzles before the curves (left). The new mobile lubrication system was installed in the 2nd test phase in such a way that 40m before the curve and immediately before the curve entry, GPS-controlled lubricant was sprayed onto the rails (see nozzles on the trolley in the right picture; source: Matthias Rogger, BERNMOBIL).
G+P carries out measurements of the trial operation for several weeks
As part of two test phases in 2018 and 2020, BERNMOBIL and Grolimund + Partner tested the new lubrication system at Le Corbusier Square in Bern-Bethlehem. Since curve noise tends to occur more often in colder weather, the measurements were carried out in November/December each year. After a successful pilot test in 2018, the first chassis of seven course vehicles was equipped with a spray system for the trial operation in 2020, which lubricates oil onto the rails before the curves in a GPS-controlled manner.
Grolimund + Partner drew on its many years of experience in analyzing noisy squeal events on railroads to measure and evaluate the more than 2,000 streetcar passes. The main objective of the trial operation was to prove that the effect of the new, mobile system is at least as high as that of the existing lubrication systems.
Figure 2: Typical level plot of two streetcar passes, each without noticeable curve noise (black) and with clear curve screech and wheel flange sizzle (red). During streetcar passes without noticeable curve noise, the 4 bogies of the 7-car articulated railcars are clearly visible as relatively symmetrical, uniform amplitudes. In the case of streetcar passes with noticeable curve noise, the bogies are no longer so clearly visible, as often several bogies emit noise at the same time.
G+P proves noise-reducing effect
The analysis of the acoustically complex situation by Grolimund + Partner provided valuable insights. Two clearly distinguishable noises were detected: a high-frequency curve screech (streetcar squeal) and a lower-frequency wheel flange squeal (listen to the curve noises in the recording above!). Fortunately, the mobile spraying of the streetcar tracks against both curve noises shows an effect: the share of noisy pass-bys has decreased by almost half, so that only every 6th streetcar emits significant curve noises. The proportion of high-frequency curve screeching, which is unpleasant to the human ear, has been greatly reduced. But not only the squealing of the streetcars has become less frequent - the volume has also decreased by a factor of two to three.
Conclusion: The new lubrication system has a future
In order to put an end to annoying cornering noises, many transport companies are increasingly investing in research into noise-reducing lubrication systems - including BERNMOBIL. Grolimund + Partner supported the Bernese public transport company by investigating the acoustic effect of a new, mobile lubrication system in tight streetcar curves. G+P was able to measure the various noises of the complex noise situation and gain valuable insights: The mobile lubrication system has potential - not only in reducing the maintenance effort, but also in curbing the unpleasant curve noises.