Rail dampers are elements that are fixed on the side (normally on both sides) of the rail and some types also have a part under the rail. Discrete rail dampers are placed on the rail at periodic distance, usually between every sleeper. Continuous rail damper are placed along the whole length of the rail, a configuration that is not often used.
The principle of rail dampers is the following: The aim is to reduce the oscillation of the vibrating rail by coupling it to a mass (steel elements in the damper) by a damped spring (rubber between the rail and the steel parts of the damper). The energy of the vibrating rail will flow into the damper (the mass of the damper will vibrate) and in turn this energy will be dissipated by the damping characteristics of the rubber. The effect of the damper on the rail is similar to increasing the damping factor of the rail which as a consequence increases the TDR.
The oscillating frequency where the flow of energy from the rail to the damper is possible depends on the stiffness and on the damping coefficients of the rubber. Changing the coefficients (rubber type) and the design it is then possible to shift or spread the working frequencies of the damper and optimize the dissipation of the transferred energy. Therefore, different designs of dampers have effects at different frequencies.
In reality the design of such a damper is quite a bit more complex than it looks. Problems arises from the fact that rubber is a material where stiffness and damping characteristics depend strongly on load, frequencies and on temperature (recent experience shows that temperature probably has a much larger effect than originally thought). To take these effects into account it is expected that there is still potential for development in the rail damper market.