To those familiar with such nomenclature: Can you refer to the contact a rail has with a metal wheel as “tread contact”? I’ve looked in railroad glossaries and found no such term; other dictionaries say that “tread” is something characteristic of rubber wheels only.
Also, when you look at the cross section of a rail, is the top referred to as the rail’s “head”? Could it also be a “cap”?
Thanks in advance!
Top of the rail is the head; I’ve never heard cap.
http://www.railway-technical.com/X-Section-of-types-of-Rails.gif
Tread is used for the outer ring of a wheel, See, for example, Lubrication, Flange on this page. The tread touches the rail in the contact zone. Scholarly references.
Thank you SO much! I’m editing an article on the topic of rails, and this is a great help! 
Glancing at the reference, an interesting question came to mind:
How quickly did railroad technology rise up the learning curve on the physics of railroad trackage and wheels? Was there a fairly long period of time in which the technology was used because of the inertia of “whatever works” and a lot of trial and error? Or did the physics of wheel/rail contact become fairly quickly known?