I finally pulled the trigger. The first attempt at laying the ring rail wandered too far from the middle of the ties, and so, this evening I pulled it all off for a redo.
Despite several encouraging comments that the prototype probably wasn’t perfect either, and it would probably look fine once the ballast was in place, I was simply not happy with the appearance. I find that a model can only take so many aspects that I’m not happy with before I completely lose interest in it, and find something else to do. So, it’s better to cut this approach off now before more compromises pile up.
The offending parts came up a lot easier than I’d anticipated. The rail itself was easy to remove with the dodgy old soldering iron. The ties only had a bare minimum of CA on them, and consequently, they popped up almost too readily. I lightly sanded it and repainted.
For setting the ties this time, I came up with a new scheme. I taped a piece of paper to the end of the bridge, aligned with the end of one of the good ties. Then for each tie, I made a mark lined up with the edge of the paper; I then advanced the turntable by one tie and repeated the process. I’m hoping the result will be a set of marks that are all the same distance from the centre of the pit, rather than from the wall.
The marks will all get covered with ties, and so, I’m not concerned about their appearance. I confess I already feel better for having corrected this. Now I can sleep!
Pembroke:87 
I was curious what your next step with the ring rail would be and enjoyed following along on this second pass. In the past, when I’ve built turntables, I’ve always found myself separating the bridge construction from the pit construction and think I like your much more closely integrated approach.
Thanks for the updates.
/chris
Thanks Chris. You’d better hold off on liking my approach until we see how it turns out 😉