Comment by mc32

Most rail profiles are like:

  |\           /|
 _| |_       _| |_

But BARTD's are more like:

   _           _
  | |         | |
 _| |_       _| |_

Correction: the rails are flat, it's the wheels that are also flat. So pretend the above are the upper half of the wheel profiles.

End result is more surface contact = more noise/squealing, also less self-centering of the wheels on the tracks, so the lips also brush against the rails. I think this also exacerbates the washboarding of the rails.