using gpx, just edit the gpxtrack so it looks like this:
[TrackManager]
TrackSet=gpxtrack.ini
CatchRecords=0
CatchMenuStrings=1
[Wad]
EnableTrackSpecificcarsWad=1
EnableTrackSpecificanimWad=1
CarsWad01=cars.wad
CarsWad02=cars.wad
CarsWad03=cars.wad
CarsWad04=cars.wad
CarsWad05=cars.wad
CarsWad06=cars.wad
CarsWad07=cars.wad
CarsWad08=carsnt.wad
CarsWad09=cars.wad
CarsWad10=carsnt.wad
CarsWad11=carsnt.wad
CarsWad12=cars.wad
CarsWad13=cars.wad
CarsWad14=cars.wad
CarsWad15=cars04italy.wad
CarsWad16=carsnt.wad
CarsWad17=cars.wad
AnimWad01=anim.wad
AnimWad02=anim.wad
AnimWad03=anim.wad
AnimWad04=anim.wad
AnimWad05=anim.wad
AnimWad06=anim.wad
AnimWad07=anim.wad
AnimWad08=animnt.wad
AnimWad09=anim.wad
AnimWad10=animnt.wad
AnimWad11=animnt.wad
AnimWad12=anim.wad
AnimWad13=anim.wad
AnimWad14=anim.wad
AnimWad15=anim.wad
AnimWad16=animnt.wad
AnimWad17=anim.wad
[Misc]
DisableBelgiumWarp=0
and then make a new cars.wad called cars04silver.wad (ie. see italy Track15)
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AMD Athlon 64 3200+, 1024mb PC3200 GeIL Goldern Dragon RAM, ATi Sapphire X800 256mb, Windows XP