Which of these vehicles must always stop before crossing railroad tracks?

Tank trucks marked with hazardous materials placards must always stop before crossing railroad tracks due to the potential danger associated with their cargo. These vehicles transport materials that can be highly flammable, toxic, or otherwise hazardous. Stopping before the tracks allows the driver to assess the situation, ensuring that it is safe to cross and minimizing the risk of an accident involving a train.

In contrast, other types of vehicles listed, like passenger cars or motorcycles, do not have the same strict regulations for crossing railroad tracks and are not required to stop unless specific conditions warrant it. Buses fall into a separate category; while some types of buses must stop before crossing, not all do. It's important for drivers of tank trucks marked with hazardous materials to follow these regulations to enhance safety for themselves and other road users.