Introduction:
The gauge in railway tracks refers to the distance between the running faces of the two rails. It is an essential parameter that determines the stability and smooth operation of trains. This distance is standardized to ensure compatibility and interoperability between different railway systems.

Explanation:
The correct answer to the given question is option 'B', which states that the gauge is the distance between the running faces of the rails. Let's understand this in detail:

Definition of Gauge:
Gauge is defined as the distance between the inner edges of the two rails on the same track. It is typically measured at a point where the wheels of the train make contact with the rails. The gauge is crucial as it ensures that the train wheels fit properly on the track and provides stability during train operation.

Reasoning behind the Correct Answer:
The running faces of the rails are the surfaces on which the train wheels roll. These faces are designed to have a specific profile and curvature to facilitate smooth movement and minimize wear. Therefore, the gauge is measured between these running faces to ensure that the wheels align correctly and do not wander off the track.

Incorrect Answers:
Let's briefly discuss the incorrect options and why they are not the correct answer:

- Option 'A': Centre of centre of rails: This distance refers to the distance between the centers of the two rails. It does not provide an accurate representation of the track width and could lead to misalignment of the train wheels if used as the gauge.

- Option 'C': Outer faces of rails: This distance refers to the outer edges or surfaces of the rails. While it is an important parameter for track design and construction, it does not represent the actual gauge. The running faces of the rails, which are slightly inward from the outer faces, are the surfaces that determine the gauge.

Conclusion:
In conclusion, the gauge in railway tracks refers to the distance between the running faces of the rails. It is a critical parameter that ensures proper alignment and stability of train wheels. The gauge is standardized to enable interoperability and compatibility between different railway systems.