What does Black Beauty fearful of at first on the Squire Gordan’s farm?

From the text:

I shall never forget the first train that ran by. I was feeding quietly near the pales which separated the meadow from the railway, when I heard a strange sound at a distance, and before I knew whence it came—with a rush and a clatter, and a puffing out of smoke—a long black train of something flew by, and was gone almost before I could draw my breath. I turned and galloped to the further side of the meadow as fast as I could go, and there I stood snorting with astonishment and fear.