Hello Zach (and JONNY)
The R-15 was one of the IRT Division cars I had in the lineup for production with my working with MTH on all the NY Subway sets since 1999. It was in the development plans before the R-142 project, which was bumped forward and produced instead. It would have likely been the following production car, along "maybe" with an R-10....had things worked out better with the US Economy, heh.
However, as the economy was slowing and tanking very badly since 2009 and hobby sales nationwide were in an ever increasing large slump, and massive US unemployment, with many hobby stores slowly closing - the R-15 project, which would have required ALL NEW expensive die tooling for the body sides and roof, (as the car uses an already existing R-17 end face)....was "shelved on hold".
And since then, as we well know, MTH has done no new tooling for "new" subway sets, and neither has LIONEL for that matter. Other than re-issues of a small few of the long existing long ago produced NY subway set products, or repaints of same.
Hopefully in the future....who knows.
---- JONNY ---- I have a few interior photos of some of my EL and subway car models -- let me look for them and post them later after I locate them in my thousands of photos.
Regards - Joe F