Letter to the Editor

Sir – I read the article ‘Driverless trains to support a prosperous future´ (RG 4.08 p225) with much interest. However I found it disappointing that there was a lack of information about any environmental impact mitigation measures for the new metro. In particular, there was nothing about the transmission of noise and vibration to properties adjacent to the viaducts on the overhead sections, or located above the tunnels where the lines run underground.

In Hong Kong during the 1990s, a great deal of effort was applied to make KCRC´s West Rail line ‘the world´s quietest railway´. As far as I know, this target was achieved. But in order to do so, specialist acoustic engineers were brought in to develop an integrated system, with parapet-mounted noise barriers and plenum walls below the walkways on the viaduct structures and rail-height noise barriers mounted alongside the track. In addition, a very resilient trackform design was used to reduce structure-borne noise, with floating slab track and highly resilient Köln Egg baseplates. Deflection of the running rails by as much as 7 mm under full dynamic loading was calculated.

On the rolling stock side, the wheel/rail interface was optimised and ‘skirts´ below the body panels were used to cover the wheels and bogies in order to reduce air-borne noise.

In the tunnels, Low Vibration Track has been used extensively to reduce ground-borne noise. Floating slab track was also installed on some of the KCRC East Rail extensions, where the line passes through particularly sensitive areas (RG 2.06 p74).

All these measures ensure that noise levels inside the trains are also minimised.

There is no indication from your article that any of these technologies have been adopted for the Dubai Metro, or even that noise and vibration has been seen as a concern. If all properties in Dubai are double-glazed and air-conditioned, then some measures may not be required. However, ground-borne noise cannot be mitigated in the building structure; it can only be achieved through the use of a very resilient trackform.

The photo on p225 shows a direct fixation system, and my concern is that this may only provide very limited attenuation of the structure-borne noise. Radiated noise from the structural steel shells at the stations could also be a cause for concern, unless the stations are isolated from the viaduct structures.

Michael I Baxter
Track International Consulting
Hong Kong