The largest percentage of material in a rail is located in its head.

Explanation:
- The rail is an important component of railway tracks, which provides a stable and durable surface for train wheels to run on.
- It is typically made of steel and is designed to withstand heavy loads and constant wear and tear.
- The rail consists of three main parts: the head, the web, and the foot.
- The head is the uppermost part of the rail, which comes into direct contact with the train wheels.
- The web is the vertical section that connects the head and the foot.
- The foot is the lowermost part of the rail, which is embedded in the track bed.
- Each of these parts serves a specific purpose in distributing the load and maintaining the stability of the rail.
- However, when it comes to the percentage of material, the head contains the largest portion.
- This is because the head is subjected to the most wear and tear due to direct contact with the train wheels.
- It needs to be strong and wear-resistant to ensure the longevity of the rail.
- Additionally, the head is wider and thicker compared to the web and foot to provide a larger contact area for the train wheels.
- This helps distribute the load more effectively and reduces the stress on the rail.
- The web and foot, although important for the overall structural integrity of the rail, do not require as much material as the head.
- Therefore, the largest percentage of material in the rail is in its head.

In conclusion, the head of the rail contains the largest percentage of material.