You can convert it to the hi-rail wheels, but it's not necessary on FasTrack unless you have turnouts. An F40PH is essentially a Geep and my Geeps worked fine running on FasTrack for our demo layout. They're not designed for curve-replacement 3-rail turnouts (O-72, etc.) as the frog has the curve continuing through it and the scale wheels will want to pick it. They seem to work fine on #5 Ross and Atlas turnouts. We have a #8 Curved turnout on the layout in which we installed a "frog point" because the frog gap was so large even a couple of 3-rail cars would derail on it -- the scale-wheeled cars and locomotives hated it.
Basically, you need "flat-top" rail to handle scale wheels. This can be Gargraves/Ross, Atlas, ScaleTrax or RealTrax/RiteTrax.
I have 15 scale-wheeled MTH engines and have run all of them on the club 3-rail layout (Gargraves and Atlas 3-rail track w/Ross turnouts) and they run fine. They WILL find the bad spots in the track work, though (out-of-gauge, bad kinks, twisted track.)