Which type of trains must be stopped and inspected upon receiving a detector failure message?

Key trains are defined as those with specific criteria that categorize them as high-priority for safety and operational reasons. When a detector failure message is received, it indicates a potential safety issue, such as an overheated wheel bearing or other mechanical concerns that could jeopardize the train's operation. Since key trains typically carry hazardous materials, valuable cargo, or are essential for the timeliness of transportation services, they must be stopped and inspected immediately to ensure safety and compliance with regulations.

While other types of trains, such as freight and passenger trains, may also require some level of response to detector failures, the immediate inspection is critically emphasized for key trains due to their potential implications on public safety and environmental concerns. Thus, the protocol for key trains reflects the heightened level of scrutiny necessary for these significant transport assets.