I was with Simon above, and presumably Joe as well. A hobby is for enjoyment. You can do what you want, within your personal budget - that includes as many rails as you want, and on the mantel forever if that makes you happy.
I don't think "Proto-48'ing" "Old Rivets" would be that hard. I wouldn't do it because my narrow gauge track cannot handle the .115 wheel treads, but I think a little lathe work and some shaft loctite would do it. I personally would put a chain in there to drive the idler axle.
On the Lionel - these things are so stunningly done that it makes the track, wheels, and couplers incongruous. I have 2-railed the Lionel FEF, and it is three times the model that its USH neighbor is. I did use a Lobaugh frame and USH drivers (and accurate side rods) but the rest is pure Lionel. Somebody else has the magic electronics.
I have the Lionel 4-12-2 as well, and judging from all the enthusiasm generated by my ad on the Buy/Sell forum, I will be converting that someday too. Its big flaw is the sand dome, but I own a Dremel tool . . .
