The technology of mobile rail milling has developed over the last 20 years to become a mature and proven method of rail head treatment. Across central Europe, the process has been accepted as part of a rail engineer’s ‘toolbox’ and has been used effectively to improve rail head management.
During that 20-year journey, Rail Engineer has looked at the technology on several occasions, most recently in issue 84 (October 2011).
Over past years, several rail millers have been at work on DB Netz infrastructure in Germany. However, as the German infrastructure owner has now spent several years maintaining its network regularly, rail head management has stabilised, meaning the demand for rail milling is actually decreasing at the moment.
This all benefits the UK’s railway, as one of Strabag’s SF02 road-rail milling units, previously used in the UK and the subject of Rail Engineer’s 2011 article, has been freed up to work on Network Rail’s infrastructure.
Bringing the SF02 to the UK has been made possible by close collaboration between Strabag and Network Rail’s profile treatment team, which is also responsible for the rail grinding fleet of trains and support services.
In fact, the Network Rail team sees the introduction of milling as being complementary to the rail grinding service already on offer. Milling enables more metal to be removed, which helps restore the rail profile. This will then release the fleet of grinders to carry out regular preventative maintenance around the network.
The EAC (Engineering Acceptance Certificate) approval process was supported by Aegis Engineering Systems, whilst ongoing operational support is from TES 2000 and Bakerail Services. There is a close working relationship with Network Rail’s route teams to identify sites that need to be treated, which is then followed up with inspection to confirm suitability.
Having obtained engineering and product acceptance at the beginning of 2016, the road-rail milling machine has completed 23 shifts across the UK, from Scotland to Wales. This geographical flexibility stems from its ability to move between sites in road mode before being quickly set to work on track, without requiring a train crew or routing to move it around the rail network. So far, 16,756 metres of milling (single pass) has been carried out while improving 7,844 metres of track, factoring in multiple passes.
The rail milling process
At the heart of the process is the milling cutter head. Fitted with paired rows of tungsten carbide cutter tips, it rotates at high speed, generating thumbnail-sized pieces of swarf which are contained within an enclosure and recovered for recycling. This creates a safe working environment for those working around and alongside the machine. The rail profile is generated by the shape of the cutter head, not the individual cutter tips, resulting in a consistent finish to close tolerances.
The cutting process leaves small facets on the rail head which, if not treated, would result in a low- pitched rumble from the wheels of rail vehicles passing over the track. Consequently, a grinding wheel, set at 10 degrees across the rail head, removes the facets on the rail – in the wheel contact patch area – resulting in a final surface roughness of around 3μm. Once again, the grinding wheel is mounted within an enclosure, allowing any dust to be recovered and containing the small stream of sparks generated by this light grinding, significantly reducing any potential fire risk.
The guidance system of the milling unit is such that any existing rail head imperfections, such as longitudinal or transverse ‘waves’, whether long undulations or short corrugations, are removed completely.
As a process, rail milling is very clean and, with an operating noise level of 82dB, provides a good working environment for operatives and doesn’t annoy lineside neighbours.
Effective recovery of milling swarf for re-cycling is a significant environmental ‘tick in the box’, whilst the collection of grinding dust prevents both health issues and any effects on signalling and other lineside systems.
The machine being used in the UK was built in 2010 by Linsinger of Austria. The tractor unit is based on a MAN vehicle while the trailer is purpose-built and contains all the control systems together with a self- contained power source for the milling and grinding operations.
Not being rail bound, the unit has the flexibility to move around the country on the road network. However, it does require a suitable on-tracking location to enable it to transfer from road to rail and back again.
The operation is entirely self- supporting with a crew that both operates and maintains the vehicle, backed-up by a well-equipped mobile workshop in a separate truck that travels with the machine.
Rail milling is not a replacement for rail grinding. This is a common miss-conception. Rather, it is complementary both to grinding and to other initiatives that aim to improve rail life and reduce the effects of rolling contact fatigue (RCF) such as improved grades of steels, friction modifiers and preventative profiles.
By removing between two and four millimetres of material, milling can recover rail damaged through cracking or plastic flow, restoring it to ‘as new’ condition having removed surface defects and re-set the rail profile, so extending its life. In some cases, by the effective application of milling, and with interim control through such measures as preventative grinding and friction management, the life of a section of track can be more than doubled.
Since the SF02 was introduced onto the network, a number of problematic areas have already been tackled.
The machine undertook its final trials for product acceptance on the Timperley and Garston lines near Warrington, a heavily used freight route for coal, and as a result of the good work carried out has since returned. One of the earliest identified applications was the treatment of heavily damaged low rail with plastic flow and head defects. There was concern that the condition of the track could damage wheelsets and make train passage noisy and uncomfortable. Whilst not unsafe, the rail still required up to six passes in places to recover the profile. Since its initial visit, the machine has recently returned to treat further sections and avert the significantly costlier process of re-railing.
Once its EAC had been issued, the first site to be visited by Strabag’s SF02 was at Barassie in South Ayrshire.
This was a classic situation with both rail defects and poor profile. Despite initial nerves due to the newness of the process, the machine performed faultlessly and the route team was more than satisfied with the final result.
A length of track at Kingsbury had suffered rail head damage from a particularly bad wheelflat, and this initiated squat defects over some six miles of track. Initial treatment by single-pass grinding did not remove the defects due to their depth. Milling was the only option other than re-railing.
The SF02 was able to remove up to 0.9mm in one pass, an average being 0.6mm, and leave a finish roughness of between 2 and 5μm, which successfully tackled the problem.
Rail milling is a good process to use in tunnels as the milling process is clean and the reduced amount of grinding sparks minimises the risk of fires. This ability was put to good use at Heathrow on infrastructure managed by Network Rail’s Reading delivery unit. While the rail head profile has not suffered the level of deformation seen at Warrington, rolling contact fatigue up to 1.5mm deep had been measured using eddy- current measuring equipment.
Previous use of a rail grinder to remove the defects had not proved entirely successful as the machines were unable to get to the area frequently enough and, with the amount of metal to be removed, the RGH-20C switch and crossing grinders could not meet the requirement. The rail miller proved to be ideal as it removed the depth required in two passes and worked within the tunnel environment without issue.
Further shifts are now planned both at Heathrow and around the country as Strabag’s SF02 road-rail miller tackles some of the network’s most problematic stretches of track.