Squat-type Defect Mitigation on Sound Transit and The Search for Effective Preventive Maintenance Measures

by Jeff Tuzik

Figure 1. A squat-type defect shown both pre- (above) and post-grinding (below). *Photo: Jeff Tuzik*

One of the most vexing, and increasingly common rail defects that rail transit agencies contend with is the squat-type defect (stud). Addressing studs has been challenging in that they are not fully understood, and, while contributing factors are known, the exact root cause(s) are not definitive. “Identifying and classifying the cause of rail damage is the first step in executing an effective prevention or remediation program,” said Mark Reimer, Director and Co-founder of Sahaya Consulting, at the 2025 Wheel/Rail Interaction Rail Transit conference. Reimer and Sahaya Consulting’s experience at Sound Transit, where rapidly forming studs have been the focus of maintenance efforts for many years, have helped build a better understanding studs and stud remediation, but questions about how to prevent them in the first place remain.

If caught early, many rail defects can generally be controlled by regular preventive grinding. As defects grow deeper, remediating them requires significantly more effort and may require rail milling to remove more metal, or even rail replacement. Studs are problematic because they are known to develop within 10 MGT, and by the time they are visible they are generally quite large subsurface defects—too deep to remove through a typical maintenance grinding program, Reimer said. Figure 1 shows an example of a stud pre- and post-grinding. When multiple studs occur in close proximity, it’s common for the defects to link up beneath the rail surface and cause large pieces of material to spall out, he said.

Figure 2. A summary of typical characteristics of squats and studs. *Courtesy: Stuart Grassie*

Although studs are called “squat-type defects” due to their appearance, “the causes and consequences of these defects are very different,” Reimer said. Figure 2 summarizes some of the known characteristics of both squats and studs. Perhaps the most important among their differences is that studs can develop within 10 MGT and that unlike squats there is no evidence that studs can develop into a transverse defect or broken rail. “There doesn’t appear to be any critical safety implications with studs [as there are with squats],” he said. “And although there have been rail breaks at or near studs, they don’t seem to have occurred because of the studs.”

Based on data from multiple studies and multiple transit agencies studs are known to have the following characteristics, Reimer said:

Figure 3. A macroscopic image of the thin martensitic while-etching layers present on the rail surface above subsurface studs.

There is little to no plastic deformation.
They can form very quickly (within 10 MGT or less).
They have been found in both new and old rail, on tangents and in curves.
They always form in the presence of white-etching layers (WELs) a martensitic layer which is very hard and brittle—evidence of a thermal change such as a wheel slip or grinding activity (See Figure 3).
They do not appear to form in tunnels.

Based on these characteristics, and experience on Sound Transit and other transit agencies, there are several contributing factors that appear to be related to stud development. One contributing factor is the use of premium heat-treated rail steels. “Light transit systems don’t see the same level of wear as freight railroads do, so the white-etching layers that form aren’t worn away quickly,” Reimer said. This may also be why studs don’t seem to form in tunnels, where the drier environment leads to higher rail wear rates and more consistent friction levels.

Another contributing factor is the use of anti-head check rail profiles, which undercut the gage corner and concentrate more contact stress on the top of the rail, which is where WELs tend to form, he said.

Traction effort has long been thought to be a contributing factor in stud formation, as well. The switch from DC to AC traction, and the addition of multiple driven axles that has occurred over the last ≈30 years coincides with the appearance of studs on many transit systems. “The significant increase in tractive effort over the last 30 years may be a contributor, but we’ve also seen studs on a system with older DC traction, and on systems with linear induction motor systems,” Reimer said. So, this remains a working theory rather than a smoking gun. Another theory is that studs may be initiated wheel slips on work vehicles, rather than revenue service fleets; there is no conclusive evidence at this time.

Studs On Sound Transit

Figure 4. A macroscopic image showing the type of crack propagation typical of studs.

When studs first began to appear on Sound Transit’s system, the primary concern was that they were squats that posed a significant risk of a rail break. But data from studies conducted by a number of experts at Sound Transit and at other agencies convinced all involved that the defects they were seeing were in fact studs and not squats, and that they posed no heightened risk of a rail break, Reimer said. Figure 4 shows a figure from one such study—a metallurgical analysis of a stud, which highlights the way the defect propagates horizontally and comparatively shallowly, rather than vertically though the rail, unlike defects that are likely to cause rail breaks. “Instead of turning downward into the rail, most of cracks we’ve sectioned start to migrate back to the surface,” he said.

The first location with a high occurrence of studs was a tangent track with co-occurring corrugation. Sound Transit approached the problem by remediating the studs in small batches, Reimer said. “We did weld head repairs, then moved on to rail grinding, then rail milling, and now there’s an ongoing rail renewal program in place to get the studs out of the whole system,” he said. Part of this work has also been to better understand studs and figure out a way to prevent them before they occur, or very early in their development. Sound Transit has since phased out rail head weld repairs, as some of these repairs have led to rail breaks in conjunction with stud defects.

“We haven’t seen a stud develop into a rail break, but that doesn’t mean that rail breaks never happen around them; only that the stud itself is more a maintenance issue than a safety issue.” Reimer said.

There is one particularly stud-affected five-mile stretch of track where Sound Transit has done much of this work(although studs appear elsewhere on their system). “Initially, Sound Transit considered that this stretch was simply the result of defective rail,” Reimer said. But as work continued, studs were found in all types and ages of rail used on the system. Additionally, tests of stud-affected rails showed no inherent metallurgical defects, disproving the “bad rail” hypothesis, he said. As the effected rail has been replaced over time, studs have continued to appear, though lesser in number and severity, he said. Nonetheless, Sound Transit has, over time, replaced many of the most severely affected rail sections.

As a method of removing studs, even at the incipient level, rail grinding proved to be inefficient on Sound Transit, Reimer said. “Through rail grinding, we exposed the subsurface cracks of the studs, but didn’t get to the bottom of them.” To remove these studs, multiple passes of a rail milling machine were required to reach their 4-mm to 8-mm depth—an amount of metal removal that is unfeasible for a grinder, he said.

Since the rail was milled, it has accumulated ≈15 MGT and there are no signs of redeveloping. The milled and the replaced track sections have also been ground regularly, beginning in 2018, he said, which likely contributes to the lower incidence of studs.

Figure 5. Eddy current measurements of stud severity (the blue bars) versus severity estimates based on visual inspection (red dots).

As part of these preventive grinding efforts, consultants have used eddy current (EC) and ultrasonic (UT) technology to help “identify, characterize, and map out stud locations,” Reimer said. Figure 5 shows an example of measured stud severity (the blue bars) versus a visual inspection of severity estimate (red dots). “The problem with studs is that they don’t always look as severe as they are until you start cutting into them,” he said. “We’ve had mixed results using both eddy current and ultrasonic technology to map out stud severity, so this is a work in progress.” It’s even more difficult to map or measure stud severity in their incipient stage using eddy current or ultrasonic technologies, so visual inspection is still an important part of the toolkit, he said.

Despite the remediation (rail milling and rail replacement) and preventive grinding efforts Sound Transit has put in place, there are still challenges with measuring and preventing stud development. And, the true root cause is still unknown. “We’re pretty confident that the presence of WELs is a key indicator of the root cause, “but we don’t know how to measure the thickness or depth of WELs, or at what depth or thickness it might initiate a stud.”

Sound Transit has implemented friction management in the form of gage-face lubrication to address noise concerns, but, Reimer said, the addition of top-of-=rail friction modifiers could potentially reduce wheel-slip events and thus reduce stud formation.

Sound Transit’s struggle with studs is a work in progress. While progress has been made in remediating and slowing their development, their root cause remains unknown. The understanding of squat-type defects has also evolved over time. And with studs becoming more common on transit systems around the world, this understanding is sorely needed. Still, knowledge gaps remain on how to prevent stud formation, rather than simply remediating them; “Prevention is always preferable to remediation,” Reimer said. The industry is working on it.