What does a K-card generally signify in train operations?

A K-card is specifically associated with block limits, serving as a vital tool in train operations to ensure safety and effective management of train movements within defined segments of the rail network. The K-card typically indicates the boundaries within which a train must operate, thereby helping maintain proper spacing between trains and preventing collisions. It plays an essential role in enforcing operational protocols that dictate when and where trains can travel, particularly in areas where track conditions or other factors might require additional caution.

While the other options represent important aspects of railway operations, they do not directly align with the primary function of a K-card. Emergency procedures focus on actions taken during a crisis, operational guidelines pertain to the overall framework for train operations, and schedule modifications relate to changes in train timings. None of these directly embody the specific responsibility of defining and managing the block limits that the K-card signifies.