I did not see this coming. Apparently Kato now has concrete tie flex track, in stock at that train store we dare not mention.
Kato now has N FLEX TRACK 31"3/16 10PC and N CONCRETE TIE FLEX 10/BOX along with track nails, terminal joiners, joiners, and insulated joiners to complete the package. Gives you some "Flexibility" using unitrak products. Don't know the specifics on matching roadbed height between the products, but I'm sure there's a way to get things to match, probably with proprietorial products to start with. John
The regular flex track and accessories have been available for quite some time now, but the concrete tie flex track is brand new.