The Super Street "Y" is more like a switch. The points are the part of the switch that move to route the train one way or the other. In my situation, the points do not genteelly blend with the rails and the taxi wheel hits the leading point edge and derails. Speed was an issue for me too, but it didn't make the problem go away. Visually, after many "practice" derailments I was able to identify the issue and evolve a solution. I used super glue to attach the shim.
The taxi wheel base plays a role here too. The longer the wheel base the more parallel the vehicle is to the rails. With a short wheel base the vehicle can be slightly cocked in the rails especially as it enters the "Y" or maybe in your case the crossing. The slight cocking directed the vehicle hard against one rail when entering the "Y" and the wheel flange would collide with the leading edge of the point.
