I'm sort of into trains....but not a geek! My claim to fame is having 3D printed one of the most famous railway bridges in the world. Here's a YouTube video of it:
https://www.youtube.com/watch?v=fyxZTt8yv80
The loco you are referring to is A4 No 4496 Dwight E Eisenhower, one of the six remaining Gresley A4s built for the London & North East Railway in Britain. And you're correct that it came back home for a reunion with its five equals....one of which, Mallard, holds the world speed record for a steam locomotive at 126 mph, which was achieved in 1938...and still stands today!
Regarding your question....I think you're not going to easily get rid of the fillet of solid infill. A supposed "improvement" in the solid infill that S3D introduced in v4.1.0 introduced some weird pieces of infill that didn't exist before the upgrade. I'll have a play around tomorrow and see if I can improve things....