I recall reading in an old issue of Model Railroader an article about railroad bridges. In it, the author mentioned that you shouldn't have a model suspension bridge for a railroad track on your layout because no such arrangement would be prototypical.
In his own words, "suspension bridges and trains don't mix."
I realize that the author probably wasn't an engineer (at least in the sense of this site), but his remark did get me thinking. I am somewhat of a railroad buff, and I can't think of a suspension bridge for trains - the closest thing that comes to mind are the trolley tracks that were on the Brooklyn Bridge*.
I understand that there wouldn't be many suspension bridges for trains, since other bridge types would be better suited (cheaper, easier to build, etc.) but why are they so rare?
*Bear in mind, though, I live in the US, and am pretty unfamiliar with foreign railroad infrastructure.