Like the thread that was started last week giving people the opportunity to express what is missing from the RTR locomotive market, the LEGO company actually has a site where people can submit ideas for new sets.

One of the sets currently trying to make it to 10,000 votes for a shot at becoming an actual set, is a build depicting the Mt Washington Cog Railway:  https://ideas.lego.com/projects/77539

With a bit of luck and support the set could make it into production for the 150 year anniversary of the Peppersass cog engine in 2016.