You're familiar with the term "mph", right? Miles-per-hour. It tells us how many miles a vehicle is traveling within one single hour.
In this problem, it tells us how many miles the train is traveling in one and a half miles instead (1.5 hours). So we have to divide the miles traveled by the time interval, which is 1.5 hours. Doing so will give us 36.67 miles traveled in one single hour, or 36.67 mph.