Well, if there are no takers for Rocket Precision then time for another blast from the past.... and one which might be a solution to the problem of gauging 3D-print chairs without keys. In the good old days, when the trade pages were green (or some other nonchalant colour), the worthy Model Railway Constructor carried adverts for Allen Brett Cannon of London Bridge (ABC to all and sundry). Amongst the many desirable nits and pieces which could be bought from the emporium there were cast chairs for 4mmscale BH rail - and those chairs were cast with a spigot.
Given the accuracy which is possible in the drafting design and subsequent 3D printing processes then, provided that a suitable drilling jig is made, including a spigot to the underneath of a chair will enable accurate and repeatable construction of track with wooden sleepers. A second drilling jig will enable accurate gauge widening for those kitchen-table curves.
regards, Graham