I've had no problem with Gilbert Flyer on SHS track. If there is any bumping, it's not noticeable.
The rail height on Lionel's S FasTrack is the same as SHS/MTH. SHS on left, Lionel on right:
S-Trax and FasTrack are more or less compatible, a modification can be made to join them together. Info here.
And MTH made no mechanical or dimensional changes to SHS's S-Trax. MTH on left, SHS on right:
Rusty