Fast glass isn't really needed for model railroad photography. Most shots should be on a tripod, and with the lens stopped down for maximum depth of field. And low light performance shouldn't be a requirement for model photos. Real world ones, yes.
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good points, Bob. One thing to look at if you are more serious than most about mrr photography is the minimum aperture. Most photographers talk about maximum aperture (smaller f#s), but for maximizing that depth of field (dof) and therefore the realism of the shot, a smaller aperture (big f#) is much-more-better for miniatures work. One of the surprises I found when I realized this was that my very cheap ($150 new!) Canon 75-300 has a minimum aperture (@300mm) of f45! On most lenses, the more you zoom the smaller the aperture becomes. (example, on that 75-300, the min aperture at 75mm is f32, but at 300mm it is f45)
Now, if you can do a shot on your layout at 300mm and get anything but a small spot on a wall, well, you've got something.
Or a very large layout....
For not a lot of $$ you can get a pinhole lens with an effective aperture of near f100. Now those are fun!
MRR shots are usually done with no flash and in aperture priority mode with, as Bob said, the aperture set as far as it will go (big #). The camera (if in Av) will set the shutter speed where it needs so you get a good pic regardless of the light. In most cases the shutter speed is going to be very very slow, on the order of 1 or more seconds, so a tripod really isn't an option, it's absolutely required.
Another spec to look at when choosing a lens for MRR work is the minimum focus distance. Anything closer than that will always be out of focus and need to be removed to help the shot work. On that 75-300 above, the min focus is 5' so it really isn't as usable as I might wish.
A good site to look at for all sorts of depth-of-field answers is:
www.dofmaster.com
they even have an iPhone app, i use it in the field all the time.
