What type of background and writing color is used for speed signs for freight trains in metropolitan areas?

The correct answer indicates that speed signs for freight trains in metropolitan areas utilize a blue background with yellow writing. This color combination is specifically chosen to ensure high visibility and easy readability, which is critical in a busy metropolitan environment where freight trains operate. The blue background helps to effectively capture the attention of train operators, while the yellow writing contrasts sharply against the blue for clear legibility at varying distances and speeds.

In contrast, other color combinations are used for different purposes in railway signage, such as caution or stop indications. For instance, a red background often signifies danger or a stop signal, which would be inappropriate for speed restrictions. The green background with black writing generally indicates information or location signs rather than speed limits. Yellow backgrounds typically highlight cautionary or advisory signs rather than those specifically assigned to freight train speeds. Therefore, the chosen color scheme of blue and yellow effectively serves the specific requirements for signaling speed limits in a metropolitan freight environment.