An excess percentage of carbon in steel makes the rail:

In steel, the carbon content plays a crucial role in determining its mechanical properties. When the carbon percentage increases beyond a certain level, the steel transitions from being ductile to more brittle. This is because higher carbon levels result in a microstructure that is more susceptible to cracking under stress, as it generally increases hardness while reducing ductility.

In the case of rail steel, which is designed to withstand significant loads and stress from train traffic, having an excess of carbon can be detrimental. While it may increase hardness and wear resistance to some extent, the brittleness introduced by high carbon levels can lead to failures such as fractures or breaks under heavy impact or pressure.

Thus, for rail applications, the optimal carbon content is carefully controlled to balance toughness and wear resistance while avoiding excessive brittleness, which can compromise safety and functionality.