My test for "robustness" is how the track holds up on an N-trak module. Over the 27 years I've been a member of an N-trak club, what I've found is that by far the most robust track in N scale is Peco Code 55. And as Dave Vollmer points out, this has almost everything to do with how the rail is attached to the ties, not the size of the rail itself. Peco Code 55 uses a code 80 rail that is embedded in the ties so that only the Code 55 portion shows. The spike detail on this track is just for show; the real holding power for the rail is the embedded rail design. And the result is that this stuff is nearly indestructible. In the N-trak module environment, where the ends of modules get heavily abused on setup, teardown and storage, even Atlas Code 80 track is "fragile" in that the rail at the ends of the modules will come apart after about a dozen setup/teardown cycles. The spikes holding the rail just aren't up to the abuse. Ends get caught on clothing, banged against other modules, etc. But not Peco Code 55. That stuff hangs on forever. The downside is that the ties are pretty thick, although this completely disappears upon ballasting.
So I agree with Dave's addendum - if you are hand laying Code 40 rail via soldering to PC board ties every third or fourth tie, this construction will be VERY robust and very reliable. If, on the other hand, we're talking about commercial track that will have the rail held by molded spike heads that are close to "scale" for the Code 40 rail, then the track is going to be fragile, with many opportunities for it to come apart during track laying, ballasting, etc. Too bad someone doesn't make a North American prototype Code 40 rail the way Peco makes its Code 55 - that would be very robust!
John C.