Hi, many years ago I had a similar problem with my Merlin Live Steam Engines on the LGB Rad1. Problem was that the wheels back to back was to Gauge 1 Fine Scale, thus the frogs were not able to do their job correctly. I used the shim method, though in my case I used 00 Code 100 Flat Bottom rail, held in place with Track Pins via a hole in both the Code Rail and the Check rail. I did write it up in Tom Coopers Steam Lines Garden Railway Magazine No. 4 in 1986.
I prefer a mechanical fit where possible, there are just not enough adhesives that stand up long term to the garden Environment. I speak here from the bitter experience of an indoor modeler who went over to garden railways some 5 years or so ago (not me I hasten to add).
JonD