A few posts worth adding to the 21mm discussion, lifted from THIS thread.
@Mayner hits the nail on the head with that last paragraph.
We'd need a collaboration / concensus from both IRM and MM, being the two main Irish outline manufacturers, with a group of past, present and future 21mm modellers involved, to agree on the standards that would meet the needs of the many.
You'll never please everybody, but the input from experienced 21mm modellers, and just as importantly, the likes of @murphaph who are dead set on going down that path, to pave the way for anything to happen.
I still believe that it's likely to happen one day, that we will have 21mm flexible track at the very least, but when that day arrives is anybody's guess.
With the 21mm-friendly models we have to date, and the amount of open source knowledge available, combined with advances in 3D printing at home, it would seem like a big mistake for a manufacturer to NOT cash in.