What equipment is specifically fitted to the roof of a TRAILER car?

The equipment specifically fitted to the roof of a trailer car is typically the HVAC system. HVAC, which stands for Heating, Ventilation, and Air Conditioning, is crucial for ensuring the comfort of passengers in any rail vehicle, including trailer cars, which are designed to provide accommodation and amenities for travelers. In trailer cars, the HVAC system is often mounted on the roof to maximize interior space and ensure efficient operation. This placement allows for proper air circulation and helps avoid interference with other internal systems.

While surge arrestors, pantographs, and antennas can also be found on rail vehicles, they serve different functions and are not exclusively assigned to trailer cars. Surge arrestors protect electrical systems from voltage spikes, pantographs are used to draw power from overhead lines, and antennas are used for communication and signal reception. Thus, the significance of HVAC equipment in providing passenger comfort makes it the correct answer in this context.