Final answer:
Taxis are generally the vehicles not required to stop at railroad grade crossings unless indicated by signals, while school or church buses are required to stop to ensure passenger safety, and farm trucks may vary depending on local laws.
Step-by-step explanation:
The vehicles not required to stop before crossing any railroad grade crossing are typically those not specified by traffic regulations or laws. Among the options listed, taxis are generally not required to stop at railroad crossings unless there are signals indicating to do so, such as flashing lights or gates. On the other hand, school or church buses are often required to stop at all railroad crossings to ensure the safety of their passengers. Farm trucks hauling produce may or may not be required to stop depending on the state or country's specific traffic laws, but they are often subject to the same regulations as other commercial vehicles.