。
The train ran at a speed of 500 miles per hour.
Literal
Train [topic-は] one-hour [in-に] 500-mile [of-の] speed [instrumental-で] ran.
~時間に~マイル is the rate-per-unit construction: 'at X per hour.' The に marks the time unit against which the rate is measured. Note that 500 miles per hour is absurdly fast for a real train — this is a stock example sentence, not a realistic claim.