Attached Link: http://www.marietta.edu/~mcshaffd/macro/telephoto.html

Jenny, the atttached link tells a little about telephoto lenes in general.  One thing to keep in mind is the actual magnification the lens provides.  For example, magnification is different than the "zoom factor" used so much by camera advitisers.  Think of binoculars.  A  7 X 35 binocular has an objetive lens(the big one out front) of 35mm diameter. The binoculars magnify the scene 7 times.     To get the equivalent in a photo lens will take close to a 350mm lens!

50mm is considered 1:1 or close to 1x in photo use. 100mm is twice that and would give a 2x or two times larger(or "closer") view.    I don't know how far it is to the other side of the track you want to shoot but keep magnification in mind.  It takes a huge lens in mm terms to give a lot of real magnification.   And as Danny said, that generally means a drop in f stop, more light needed to get the job done.

The link might help in some general terms but is not directly applicable to the adapter you may have in mind.          TomDart.