Glad to hear you are having a good trip out west. If you are traveling via I-40, you will see plenty of the former Santa Fe transcon. If you are dropping down to Yuma and going into San Diego that way, keep an eye out for the remnant of the old San Diego & Arizona Eastern as you pass Jacumba. You won't see the tunnels and bridges from the Interstate, but you can see some of the line. Remember to keep drinking water in the car, in case of a breakdown.