What types of trains are generally referred to as special trains?

Special trains are typically those that serve a specific function or purpose beyond regular passenger or freight transportation. Work trains or lay-up trains are classified as special trains because they are utilized for maintenance, construction, or other operational tasks that are crucial for the railway infrastructure. These trains may not run on a fixed schedule and are not intended for general public use.

In contrast, the other options refer to types of trains that have more defined roles. Speed trains and bullet trains focus on high-speed passenger service, while passenger and freight trains serve the general transportation of people and goods. Maintenance and repair trains, though they serve essential functions, are often categorized under a more general umbrella but can overlap with work trains. However, it is specifically the work and lay-up trains that exemplify the specialized nature associated with "special trains."