What is the primary visual characteristic of a pressurized tank railcar?

The primary visual characteristic of a pressurized tank railcar is indeed the presence of a relief valve on top, along with the absence of bottom unloading piping under the car. This design is crucial for maintaining the integrity and safety of the cargo transported within the tank. Pressurized tank cars are specifically constructed to handle substances under high pressure, and the relief valve serves as a safety feature to release any excess pressure that may build up within the tank, preventing potential rupture or explosion.

Other characteristics like large inspection hatches, welded seams, and bright colors may be associated with various types of railcars but do not specifically identify pressurized tank railcars. The inspection hatches could be found on a range of tank cars, while welded seams are common in many railcar types. Bright colors can serve to indicate the type of cargo but are not a definitive identifying feature of pressurized tank railcars. Thus, the indicated relief valve is the primary visual indicator that confirms the specific design and purpose of this type of railcar, emphasizing a focus on safety in transporting pressurized materials.