**2. System architecture**

Catenary is composed of contact wire, cantilever, additional wire, hanging post (in tunnels), etc. (**Figure 4**). As a result, different kinds of catenary components are mounted in a large area and vary greatly in size and location.

Considering that most components concentrate at catenary posts, we use groups of super-high-resolution area-scan cameras to capture image data of each region at a specific distance relative to each catenary post. A post detection module is applied to generate an accurate trigger signal for cameras and lighting device. Finally, image data from different cameras are paralleled processed and saved in a common data sever.

The system architecture is illustrated in **Figure 5**.

**Figure 4.** *Structure of catenary.*

**Figure 5.** *System architecture.*
