As you approach the crossing the frequency you hear is f; as you recede from the crossing the frequency you hear is 7f /9. What is the speed of the train?

The Shinkansen, the Japanese “bullet” train, runs at high speed from Tokyo to Nagoya. Riding on the Shinkansen, you notice that the frequency of a crossing signal changes markedly as you pass the crossing. As you approach the crossing the frequency you hear is f; as you recede from the crossing the frequency you hear is 7f /9. What is the speed of the train?