If you look at the number of train threads in the transportation forum compared to car, motorcycle, and aviation threads, they compose a relatively small percenage of the images posted here. The big difference is that most of the people posting railroad shots are passionate about their subject. Cars and airplanes are pretty much a matter of everyday life for most of us, and we photograph them because they're accessible and somewhat common. Also, especially with cars, there is a cultural element to consider. Cars are a status symbol for many, and a goodly of the photos posted are personal reflections of the pride an individual take in their own 'ride'. Train lovers will take a picture of any train, anywhere, any time, not because it's cool or radical, but because it's a train. It doesn't make any difference what railroad, how new, how clean or dirty, how rare, etc. If it's a good shot, it's worth sharing.
Trains is trains.