Define 'rail wear'.

The term 'rail wear' refers specifically to the deterioration of the rail surface that occurs as a result of repeated train passage and the forces exerted during transit. This phenomenon is a natural aspect of rail maintenance and operation, as the constant movement of heavy trains applies stress to the rails, which can lead to the wearing down or degradation of the rail material over time.

Factors contributing to rail wear include friction from wheel contact, thermal expansion, and even wear from environmental factors such as dirt and moisture. Understanding rail wear is crucial for rail maintenance personnel, as it impacts safety, ride quality, and the longevity of the rail infrastructure. Regular monitoring and assessment of rail conditions allow for timely repairs and replacements, ensuring safe and efficient rail service.

The other options provided relate to different aspects of rail maintenance and operations but do not accurately capture the concept of rail wear. For instance, increasing rail thickness pertains to repairs rather than the wear process. Replacing old rails with new ones is a remedial action that occurs after significant wear has taken place. Painting rails to prevent rust is a preventive maintenance measure but does not involve the mechanical degradation that defines rail wear.