42" was what Lionel made in the old days, that's why it's the "standard" now. Almost all of the older tinplate will run on STD-42 curves, except for the big Boucher steamers. Some of the newer Standard Gauge items like the Lionel Hiawatha and the Super 381 either won't run on STD-42 at all or become very unhappy if you make them do it. As Chris points out, the smaller engines look best on 42" curves. Wider curves are available from USA Track, which makes 42", 57", 72", and 87" curves. I have 87" USA Track on my home layout, which is a loop of Standard Gauge around the outside of an 0 gauge layout. To echo what Chris said, come join us over on the Tinplate Forum.