I ride the train for several reasons:
For one... it's civilized. I'm not crushed in a seat, with no legroom and pretzels handed to me as a meal.
It's cost effective. 1-2 times a month I will commute from ALB to NYC for the day. Flying would be outrageous in terms of price. Driving would end up being as costly or costlier.
It's time effective. 2:20 and I'm in Manhattan. Now, admittedly the metric has changed since I now have to then go to NJ which adds about 20 minutes to my commute, so driving might actually be a bit shorter. However, I think the other drivers would be quite upset if I started napping in the car or working on my computer as I do on the train. So even if it takes a bit longer, the time is used far more effectively. Flying would be faster to the city, but then I have the haul from the airport (most likely by train anyway), which would eat into that time.
Even for longer trips it's cost-effective and time-effective. Especially if one can arrange for a trip where much of the travel time is done at night. In that case no time is really lost in a sense.
On longer trips I can shower before arrival and arrive quite refreshed. Try that on an airplane. :-)
For me, a better question would be, "why don't I take the train?" Sometimes it's not just the best option. Yet, even given the rather small size of Amtrak's system compared to "yesteryear" it often is the best option or part of the best option.
(To elaborate on that point for a second, I'll mention again my "Planes, Trains, Automobile business trip.) Pressed for time on several fronts, a co-worker and I flew from Albany to BWI, rented a car, got the Tyson's Corners, packed up a data center, returned to BWI, boarded the train to NYC, beat the servers to the new data-center, set them up when they arrived and took the train back to Albany that night. Each method of travel was optomized for its respective segment.)
BTW, on a related note, in the past year I'm personally somewhat responsible for at least one family (and perhaps a few others) including train travel in their itinerary, including a family trip from Maine to Walt Disney World. They had not previously seriously considered the train until talking to me. Once they looked, they realized for a family of four it was cost effective and quite enjoyable.
Oh and I left out all the other reasons I take the train... the scenery, the people, etc.