Wikipedia
A railgrinder (or rail grinder) is a maintenance of way vehicle or train used to restore the profile and remove irregularities from worn rail track to extend its life and to improve the ride of trains using the track.
Rail-grinding equipment may be mounted on a single self-propelled vehicle or on a dedicated rail grinding train which, when used on an extensive network, may include crew quarters. The grinding wheels, of which there may be more than one hundred, are set at controlled angles to restore the track to its correct profile.
The machines have been in use in North America, UK and Europe since the early 20th century. They are made by specialist train maintenance companies who may also operate them under contract.
The early 2000s saw several advancements in rail maintenance technology, most notably the introduction of track re-profiling by rail milling trains for which advantages in accuracy of the profile and quality of the processed surface are claimed. A second technology which is gaining widespread acceptance in Europe, Germany in particular, is " High Speed Grinding". While it cannot re-profile rails like milling or other grinding trains, its working speed of approximately 80km/h allows defect removal and prevention to be achieved with little or no impact on other scheduled traffic.