Sam wrote a tune about trains called Highballin'.  The title refers to the ball-shaped train signal that is raised to the up position when the engineer is allowed to go at top speed.  To play the tune, one starts out slowly, and the tempo increases each time through the tune.  When you get to top speed, you're highballin. 

This video is from a jam session at O'Hurley's General Store in Shepherdstown, West Virginia where Sam played on many Thursday evenings for the last 22 years of his life.