With mixed rail codes in a single fixture, you run a risk of the clearances not being correct. The fixtures are made for specific rails, and the dimensional profile of the rails are quire different in some cases. You should always construct and check with either an NMRA gage or with carefully made gage blocks and feelers for the correct track check gauge and the proper flangeway spacing. If you need to file bases of rails (or spread them away) to set the proper dimensions, then do so. Accept that the fixture is a help aide, and unless being used explicitly with the rail code designed for, not as the final arbiter for correct dimensions. It is wise to have the habit of using the gages for all construction to be 'dead on correct' every time.
Note: Rails are not always manufactured with exacting tolerances. Production runs can have deviations. Even using the correct rail for a fixture can lead to problems if the rail deviations are just off enough and checking gages are not used in the construction.
Note: I use "gage" to be the instrument (like the NMRA gage) for checking and verification. The "gauge" is a reference to a dimensional value measurment (e.g., track gauge or check gauge).
-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.