Caltrain passenger trains reach top speeds of 79mph:
http://www.caltrain.com/riderinfo/safety_security/Safety_Tip.... html
Now, I know you said "average speed", and that's probably a lot less depending on which train is running (limited stop, express, "bullet", local).
The BART (Bay Area Rapid Transit) "trains" also top out at about 80mph but average only 33mph: