Tim Brien said:
Jon,
the Aristo wide-radius will usually 'split' a switch if the divergent section is preceded by a curved section, or the wheel back to back is a little wide. The problem, as stated, is that unlike in smaller gauges, the divergent point rail is not recessed into its adjacent rail. The splitting never occurs on the straight through route. The modified frog is part of an eventual upgrade on the entire switch (hopefully).
The switch is proceeded by a long straight, but the B-B could be suspect. I haven't done any wheel adjustments to any of my 1:20 stock yet. They are being run as-is straight from the box. The locos are usually the ones that start the split, and they can't easily be adjusted. When my live steam Shay splits the switch it's a real problem because the universal/slip joint linkage comes apart. Fitting that back together on a hot engine is no fun.
As you said, the point rail does not sit inside the stock rail when the switch is set to the diverging route. If the wheels happen to be up against that rail when going through the switch, they will pick the point and bump over it to the straight route. If I hold the points tight against the stock rail with a tool, even my heaviest locos will go through with no problem. That’s why I thought changing the throw to something more solid may help.
I have tried filing on a different switch and made things worse, so unless I can figure out exactly where to trim, I’m not going to do that again.
Another quick fix might be to put a guard rail in front of the switch on the side of the diverging route to pull the wheels away from the point. While not very prototypical in appearance it might stop the derailments. I wonder if that would just move the problem to the straight route.