in reducing wear, noise and vibration, and managing wheel/rail interaction on rail transit systems. Controlling friction has become more achievable with the advent of engineered friction modifiers and improvements to wayside and onboard application systems for both traditional lubricants and friction modifiers.

Much of the benefit associated with improved lubrication/friction management materials and application systems has been gleaned from work done by the major freight railways to reduce lateral loads and gauge-face wear in curves. While the goals of transit systems (which are more sensitive to wheel/rail noise) may be different, the benefits and the net effects are the same.

Recognizing the need to make information on research and current operating practices such as these available to rail transit agencies, the Transportation Research Board (TRB), Federal Transit Administration (FTA) and Transit Development Corporation (TDC) — an educational and research organization established by the American Public Transportation Association (APTA) — established the Transit Cooperative Research Program (TCRP). A primary goal of the TCRP is to disseminate information to those who need it most — transit agencies, service providers and suppliers.

Track-Related Research Volume 4: Friction Control Methods Used by the Transit Industry is an example of the type of information collected and distributed through the TCRP. The report, which includes field reviews of vehicle-borne and wayside top-of-rail (TOR) application systems, and surveys of nine rail transit systems, shows that various types of friction management programs have yielded positive results. The report chronicles how the industry has moved from what were routinely called "lubrication" systems to "friction management" systems to reduce noise and vibration, and control wheel and rail wear.

While rail/wheel lubrication has been used extensively in the freight railroad environment to reduce wear and control curving forces, transit operators have been reluctant to apply lubricants or friction modifiers to the rail (or wheel) because of concerns about their effect on traction and braking conditions. Because of their inner city locations and use by the public, transit systems also have been more sensitive to contamination, grease buildup and other unsightly conditions typically associated with lubrication. Rail transit systems are also sensitive to wheel/rail-generated noise and wheel/rail wear that friction management can alleviate.

"The goal of friction control is to produce a specific friction level at specific locations on the wheel or rail, rather than simply to reduce friction to a low level at the gauge face," Christopher Jenks, TCRP Manager – Transportation Research Board, points out in the introduction to TCRP Report 71 Track-Related Research Volume 4: Friction Control Methods Used by the Transit Industry. "This goal requires a higher degree of system control, applicator reliability, and lubricant (i.e. friction modifier material) development."

Friction Modifiers
Friction modifiers, which are applied to the wheel tread or top of rail to produce and maintain a specific level of friction, have emerged as effective tools for controlling friction. They do, however, require a greater degree of system control and applicator reliability.

Water-based friction modifiers, such as Kelsan Technologies' KELTRACK®, produce a micron-scale film of engineered solids that leave a thin, dry film on the rail (and wheel) after the water evaporates. These engineered products, which are applied to the top of rail or tread of the wheel, are designed to establish a constant coefficient of friction, typically in the range of 0.3 µ to 0.35 µ. By doing so, friction modifiers can reduce stick-slip (and resulting squeal) while maintaining enough positive friction for normal braking and traction operations.

Tests have shown that wheel squeal is in part generated by lateral creep at the wheel/rail interface. Under certain conditions, lateral creep can lead to lateral stick-slip oscillations, which, in turn, can excite high-frequency vibrations in the wheel plate and rail. During tests in which gauge-face and TOR friction have been independently controlled, the greatest noise reduction was achieved by controlling friction at the wheel tread/top of rail.

Compared to dry rail, lubrication can reduce wheel/rail-generated noise levels by 10 to 20 decibels. This noise reduction can be achieved after only 5 - 10 vehicles pass the wayside application point. Once achieved, the noise reductions associated with lubrication can continue for several days after a lubricator is shut down.

The TRB's Chris Jenks points out that friction modifiers' ability to reduce or increase friction to a specific range, has changed the way in which the industry thinks about friction. "We've moved from 'lubrication' to 'friction control.'" This is not to say that traditional greases and wayside application systems (both of which have been modified and improved within recent years) are not part of the landscape of modern transit systems. Wayside lubrication systems continue to effectively reduce friction, noise and gauge-face wear on rail in curves.

Lubricants and friction modifiers have been applied by wayside, onboard and hi-rail systems. They have also been applied by hand (using brushes and rollers) in tests or other localized applications. Because many noise-related issues are site-specific (often identified by complaints from the public), the majority of transit operators consider wayside systems to be the most practical and cost-effective solution.