Final answer:
It generally takes around 25 seconds for a typical tractor-trailer unit to clear a double railroad track from a stop.
Step-by-step explanation:
The question asks how long it takes for a typical tractor-trailer unit to clear a double railroad track from a stop. The time it takes for a tractor-trailer unit to clear a double railroad track from a stop depends on several factors, including the size of the unit and the speed at which it is traveling. However, based on typical circumstances, it would generally take around 25 seconds for a tractor-trailer unit to clear a double railroad track from a stop.