SAUDI Arabia's North-South Railway (NSR) runs through the uninhabited An Nafud desert and is designed for heavy-haul operation with 32.4 tonne axleloads. The line, which serves the phosphate mine at Al Jalamid in the northwest of the country and The Gulf port at Ras Al-Khair, entered service in 2011.

After around a year and less than 200,000km of operation, railway personnel began to notice an unusual wheel wear pattern developing on the phosphate wagon fleet. The wheel wear problem was also appearing on locomotives and some track maintenance equipment.

Phosphate Train2As a result the Wheel Wear Task Force (WWTF), which was made up of Canarail, a member of the Implementation Supervision Consultant (ISC) consortium, and National Research Council Canada - Automotive and Surface Transportation (NRC-AST), was formed to investigate and propose solutions to the problem. WWTF subsequently collected wheel and rail profile measurements, and measured the coefficient of friction (COF) of the rails, wheel and rail hardness, phosphate wagon bogie rotation, as well as documenting wheel and rail conditions.

The line's desert location allowed the designers to use a cost-effective alignment, which consequently consists of an almost exclusively tangent single track mainline. There are very few curves less than 5000m in radius with the loading loop at the mine and an unloading loop at the port separated by roughly 1800km. However, while reducing construction and operating costs, limiting curves means wheelsets have few opportunities to displace laterally through the full flangeway clearance.

Initial operations on the railway, which commenced in the second quarter of 2011, were limited, with three trains per week operating a 3600km round trip. Trains comprised two locomotives and 120 wagons. In 2014 trains began operating with three locomotives (two at the head end and one in distributed power mode at the rear) and 160 wagons.

The 3.2MW ac traction diesel locomotives used on the line were manufactured by Electro-Motive Diesel, while CSR Yangtze supplied the phosphate wagons. All rolling stock was specified to Association of American Railroads (AAR) standards with the exception of the wheel profile and back-to-back spacing, which met UIC specifications due to track type.

High-speed stability was required for empty train operations, thus bogies designed to AAR S-286 were selected with constant contact side bearings.

After learning that the railway was experiencing high wheel wear rates, ISC engineers began making observations. ISC noted that wheels exhibited evidence of vertical flange wear on both sides, and although hollowing was present on both sides it was biased to the flange root and deeper on one side of the wheelset. Some wheels also showed multiple but distinct wear bands.

In addition the flanges on one side of the train were wearing more rapidly than on the other side of the train, while WWTF members observed evidence of wheel flange contact on the tangent rail along the NSR. However, this contact did not produce the severe wear seen on the high rails of the loading and unloading loops.

With trains operating in an anticlockwise loop they are not "turned," thus wheels on the right side of the train in the direction of travel are always running on the high rail at the mine and port loops. These loops are the sharpest curves in the system with radii of 350m (5°) at the mine and 550m (3.2°) at the port. Observation of the worn wheel profiles from some wagons show that wheels using the high rails reported the flange wear problem.

In addition, wheels with excessive flange wear developed a hollow that was biased towards the wheel flange, whereas the hollow on the mate wheels was roughly central on the tread. The depth of the hollow tended to be greater on the wheel with excessive flange wear.

High rails at the loading and unloading loops showed signs of aggressive adhesive wear on the gauge face and further investigation showed that vertical wear varied from 0.69-0.93mm while gauge face wear varied from 0.26-0.97mm. At the time these measurements were made, approximately 2 million gross tonnes had passed through these loops, meaning that head loss varied from 0.35-0.47mm/million gross-tonnes and gauge face wear varied from 0.13-0.49mm/million gross-tonnes. These rates are 18 to 24 times higher than for a North American heavy-haul railway in curves of equivalent radius that were not friction-managed. In addition, the high rails were wearing to match the shape of the wheels causing the wear.

This situation is exacerbated by desert sand. Indeed wind patterns render some sections of the line to be entirely sand-covered, further increasing wheel and rail wear rates through

NRC measured the rail's COF using a push-tribometer at both the top of rail and gauge face at various locations along the line.

The COF values are well above what Arema recommends (0.30-0.40) for the top of rail. This high COF, particularly in the mine and port curves where wheelsets are flanging with a high angle of attack means that in-plane wheel-rail forces are roughly 60-70% of the wheel load. These high forces drive the wheelset into hard flange contact causing a high flange wear rate and accelerated tread wear.


NRC was subsequently commissioned by ISC to engineer a family of new rail profiles that produce differing contact points across the wheel tread. This technique distributes wear across as much of the wheel tread as possible to delay the onset of tread hollowing and has been employed successfully on several heavy-haul railways in North America and Australia.

Two rail profiles were developed for tangent track: Contact Point Centre (CPC) profile, where the contact point is at the centre of the rail, and Contact Point Field (CPF) profile, where the contact point is slightly biased towards the field side of the rail. Two more profiles were engineered for curved track with radii less than 3500m, Low profile for the low rail, and High Rail Curve (HRC) profile for the high rail. The two tangent profiles were designed for all tangent sections as well as curved sections with a radius above 3500m with the CPF and CPC profiles used on the northern and southern sections of the railway, respectively.

To match the newly developed rail profiles, NRC also developed a wheel profile that was based on the profile of slightly worn UIC S-1002 profiles measured in the field. The flange root and tread were modified to provide better steering and reduced wear while maintaining appropriate contact stress and stability. The wheels on re-profiled wheelsets should be machined to within one AAR tape size (1mm on diameter), as a wheelset with significantly different wheel diameters will create a turning moment in the direction of the smaller wheel, and yaw the wheelset and bogie. These, coupled with high top-of-rail COF values, cause large creep forces and therefore wear.

The use of flexible bogies which incorporate cross-bracing to stabilise empty wagons at high speeds is another contributing factor to wear. This has the unfortunate side effect of the wheels travelling particularly true on the railhead thus concentrating tread wear into a narrow band on the predominantly tangent line.

An NRC engineer noticed a considerable amount of wear particles on the base of the high rail at both the mine and port loops, but only on those sections of the loops where the wagons would have been empty.

This is explained by relatively tight curves (5°), combined with a high top of rail COF, presence of sand and dust and lack of lubrication causing the wheel flanges on the leading wheelset of each bogie to aggressively rub the gauge face of the high rail.

In addition, it was postulated that constant contact side bearings may have been supplied or adjusted with a preload that was too high. If the side bearings were properly adjusted, they would carry about 38% of the wagon body weight when the wagon was empty, or 7% of the weight when the wagon was loaded.

However, if improperly adjusted and carrying, for example, a third more load than they should, they would carry 51% of the empty wagon weight, or 9% of the loaded wagon weight. The weight fraction on the side bearings would increase by 13% for an empty wagon due to the improper adjustment, but only 2% for a loaded wagon. Since wheel-rail steering forces are proportional to axleload, they may be too low to rotate the bogie under an empty wagon with improperly adjusted side bearings, but the steering forces developed for a loaded wagon might be sufficient to permit steering.

Installation of top-of-rail friction management and gauge face lubrication equipment was also recommended at the entrances of the mine and port loops. Gauge face lubrication will address flange wear while top of rail friction management will tackle the asymmetric tread wear problem and must be applied to both the high and low rails at the port and mine to reduce large frictional creep forces.

And while sand and dust will contaminate a wet lubricant with abrasive particles, even contaminated lubricant is far superior to no lubricant at all.


Improvements were also required for operations. With the railway essentially a straight line with a loop at both ends, trains travel in the same direction through both loops. This results in all wheels travelling in the same direction on the same rail in the loaded direction, and in the unloaded direction.

"Turning" the trains evens out the wear across the wheel tread, particularly where friction modifiers are not employed because it swaps the leading and trailing axles on each wagon. Leading axles, particularly the wheel on the leading axle on the high rail in curves, suffer higher wear rates due to larger tread forces acting on them than on trailing wheelsets. Turning trains repositions the leading axle, and evens out the wear. As a result it is essential that trains are turned at least every-other round trip until the wear comes under control, then once a month thereafter.

These findings from the NSR provide valuable lessons for planners of future heavy-haul lines, particularly those that will be built in dry environments and consist predominantly of tangent track. Here, utilising way-side friction management both for the top of rail and gauge face at the entrance to loading and unloading loops, developing and implementing different tangent rail profiles so that the points of contact between the wheel and rail changes along the line, increasing the clearance between rail and flange by using narrow-flange wheels, and turning trains at termini to further even out wear, are all key considerations to avoid excessive wheel-rail wear.

Additional hypotheses to explain the asymmetrical wheel wear, were put forward but ruled out through empirical testing. These included:

  • metallurgical issues with wheels and/or rails
  • mill scale
  • aggressive use of air brakes
  • wagon overloading, and
  • tight track gauge.

Instead, decreasing the friction between wheel and rail and providing two distinct but overlapping wheel contact bands on the tangent segments will greatly reduce overall wheel and rail wear and improve the lifespan of rolling stock assets.