The code 70 - 83 rail bases were not always closely the same. IN the early 1990's ME ran a custom 'pull' of code 83 rail for a client, who ended up failing to complete the transaction. ME sold off the rail by using their code 70 flex track molds, and discovered that the combination was cheaper but still effective for code 83 and continued with the new profile since then.
Code 40 and code 55 rails are significantly different in profile and always have been. In large part because they are targeted to different scales - though used often within same scales. Rial height alone does not mean identical (or even very close) rail profiles. The code 83 offerings in the market have very wildly different rail bases and webs as well as even the rail head widths.
Fixtures relying on rail bases for alignment require either custom per rail construction of the addition of rail gauges on the rail heads for compensation when constructing. While not recommended by Tim, many do in fact use the rail gauges and allow multi-rail use in a single fixture.
-ed-
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-ed mccamey-
COSLAR RR -
http://www.coslar.us/NMRA Standards and Conformance Department
PROTO & FINE Scale Coordinator
I estimate I have about 5 pounds of coupler springs somewhere in the vicinity of my workbench.