TEXAS EAGLE SPEED? - Trains Magazine

I doubt if he exceeded 79 mph.  Most if not all Amtrak locomotives carry a black box recorder that upon routine examination of the record tape would then result in the engineer's severe reprimand if not termination.

Is it possible the Interstate and the tracks were not quite parallel at the point where you observed the apparent higher speed, causing you to drive faster to catch up or match the train's speed?  For example, there are bends in I-55 near Elkhart and Williamsville, IL, but the track is still straight.

The only track of which I am aware that is signaled for 100 mph train service is along the old AT&SF mainline through Kansas and possibly Colorado and New Mexico to near Las Vegas, NM.  That's the route of the old Chiefs and the present Southwest Chief.