Final answer:
The third quartile of the distribution of x represents the value below which 75% of the time between two successive trains passing through the crossing falls.
Step-by-step explanation:
The third quartile of the distribution of x represents the value below which 75% of the time between two successive trains passing through the crossing falls. In other words, it gives us a measure of the time within which most of the train intervals fall.
To find the third quartile, you would need a dataset of train intervals. Then, you would arrange the intervals in increasing order and find the value at which 75% of the data falls below. This value will be the third quartile.