I'll offer up one of each. Riding double headed steam up the 4% grade out of Morton, WA with Jack Anderson. Sensory overload - I could feel it through the floorboards, plus the sounds, sights and smells - I don't think the smile wore off for at least a week.

Southbound from Denali Park station on the Alaska RR. When we approached the Hurricane Gulch bridge, the fireman started tearing pages out of the timetable. He told me to make a paper airplane. The bridge is on a slight downgrade south, so we brought the train to a slow crawl before the bridge and then kicked off the brakes and let it roll. Meanwhile, everyone went out on the walkway and launched their airplane. Since the bridge is something like 265 feet high, we saw our planes go under the bridge out of sight, and then reappear much lower, at least twice. My only cab ride on the ARR, but it was a great one.