What is the four-digit number appearing on a placard or an orange panel on a tank car called?

The four-digit number that appears on a placard or an orange panel on a tank car is known as a United Nations (UN) product identification number. This number is crucial for first responders and others who need to identify the hazardous materials being transported. Each UN number corresponds to a specific hazardous substance or group of substances, enabling safety professionals to quickly ascertain the type of material involved in an incident.

This identification is vital in emergency situations, as it helps determine the appropriate safety measures and response strategies to mitigate any risks associated with the material. The UN numbers are standardized internationally, which allows for consistency in identifying hazardous materials globally.

Other options do not correctly represent the four-digit number's purpose. The capacity of the tank car refers to how much it can hold, while the last date the tank car was pressure tested pertains to safety compliance rather than identifying the product. The tank car registration number is specific to the car itself and does not provide information about the hazardous material contained within.