Most Forumites are from the eastern half of the country and it's interesting to see their answers to the topic question.
In the West, railroading is different. It isn't too difficult to get to a location away from population where you can hear a train coming before you can see it, and, sometimes, you can see the train long before it gets to your location. Between trains, the aroma of creosote drifts up from the track, and the ties creak as the rail adjusts to the temperature. There's some undulating territory where trains operate at middle speeds, and a lot of territory where the locomotives are wide open, making from 15 to 50 MPH on long ascending grades, or flying past on 70 MPH freight trains, including coal trains and double stack trains almost 3 miles long.
Like Hot Water pointed out, it's a sensory experience -- the noise, the aroma, the ground shaking and the dust flying. I'm old enough to have enjoyed first generation EMD's in heavy grade territory on Santa Fe and Southern Pacific, and the whine of the cooling fans and the roots blowers -- along with what David Morgan called the chant of the EMD 2-cycle engines -- first becoming audible down the canyon, then getting louder and louder, reaching a crescendo when the engines passed, and then the chant gradually fading into the distance for several more minutes . . . well, it was stirring, to say the least, and those memories are very fond ones which keep me coming back to trackside.
And then there were the San Diegans, departing westward out of Fullerton with two Alcos that eased the train into motion and then wound up into a raspy shout while billowing black exhaust as the train accelerated. Or No. 23, the Grand Canyon, 16 mixed lightweight and heavyweight cars accelerating behind warbonnet F7's with a cab unit on each end and two or three booster units in between. And I can't forget the fragrant aroma that the dining car exuded as it passed. When I watch a modern passenger train come and go at a station, I recall how my own days as a passenger Engineer were influenced by those past memories.
There is just something about the railroad that is in my soul and being at trackside satisfies and allows me to put daily life on the back burner for a little while. That's why I still railfan after 75 years of watching trains and 36 years of working for the railroad.