DOH!
Ok.... I couldn't find anything wrong with the entrypoints visually (other than moving a few forest ones that I didn't know I couldn't have to begin with) , so I deleted them in stages thinking I'd eventually remove whichever was the "bad" one(s), at which point I'd then restore the terrain project files and determine what I'd done...
Except that I eventually removed ALL of them, and when I run the entry check, the TE still crashed immediately.
It didn't do this when I began entering spawn points, so I knew for certain I'd mucked up something else entering them.
And then I realized that if I searched for objects by Type with all spawns removed, if it found either a boat entry or vehicle entry on the list I'd probably have found my problem....
which I did.
I'd bobbled at some point and added a "delta4" shape accidentally as a "Veh Ent" type. (Probably when the train bounced {grin})
Anyway, I removed that when I found it and I'm now all set - but I wanted to thank you all for all your kind help.
Guy
EDIT -> PS. While I don't know what kind of issues this bogus entry might eventually have caused, at least in offline mode it didn't affect the terrain adversely - it would still build in the TE and run as the Offline Terrain with no issues that I could see. Of course, I don't know what would have happened if I'd tried to spawn at the bogus spawn point.... I didn't try.