Angle grinder with a cut off wheel. You'll lose hardness right on the end you cut, which can be reduced a bit by wrapping it in a wet rag, this will also reduce the chance of warping if it's a smaller diameter. But even without it will probably not be enough to be noticeable. The rails are meant to be travelled the most in the middle. The ends are typically clamped in a holder and the hardness does not matter.

Also be careful not to scratch it when you clamp it in the vice to cut.