Which type of vehicle must always stop before crossing railroad tracks?

The requirement for certain vehicles to always stop before crossing railroad tracks is a critical safety measure, particularly for vehicles that transport hazardous materials. Tank trucks marked with hazardous materials placards must stop at all railroad crossings, regardless of whether the crossing is marked with signals or not. This is to ensure the safety of both the truck driver and the public since these vehicles carry substances that could pose significant risks in the event of an accident due to a train collision.

By stopping, the driver can take the necessary precautions to ensure that no trains are approaching before proceeding. Other vehicles, such as passenger cars or motorcycles, may not have the same legal requirement to stop at every crossing unless directed by signage or specific conditions. While school buses also have a requirement to stop at railroad crossings, the focus here is on the additional risks associated with hazardous materials, making tank trucks marked as such the correct answer.