In a list of American words compared with English equivalents, Mencken gives American (railroad) ballast as equavalent of British "metal". I suspect railroad "ballast"
was an extension of use of stones to add weight just above
keel of boat to improve lateral stability.
I cannot imagine how "metal" came to mean preparation for laying railway tracks in England.

how "metal" came to mean preparation for laying railway tracks in England

From AHD4:

Metal

5. Broken stones used for road surfaces or railroad beds.

See http://www.bartleby.com/61/48/M0244800.html

Dear Faldage: What I was wondering was when metal came to be used for malleable,ductile substances suitable for tools, weapons,armor, etc.?

Ah! Have to check the OED on that one Dr Bill.