I would say, the first time I traveled by train in the United States. I had traveled by train in the UK and Europe as a teenager visiting, but US train travel is of course uhhhh not competitive, and other than the monorail at Disney World, I didn't have access to US train travel until I moved to New York about five years ago. Amtrak is very good in New York and the northeast (comparatively) and it's almost the same amount of time to drive to New York City from where I live upstate as to take the train.

The Hudson River where I live is fine, but it's huge downstate. Watching the river go by and change, and the landscape itself change as the train moved through the Catskill Mountains, for the first time was wonderful. The moments when New York City begins to appear. Going underground. And then all the train travel that came after, the local subway experiences. Magical! Even though I've done this trip many times now, and it's always very nice, that first trip will stay in my memory.