Amodel train is 6 inches the real train is 42 feet what is the scale factor in inches

1 foot = 12 inches
42 feet = (42 x 12) = 504

The factor of (6 inches / 42 feet) is (6 inches / 504 inches) = 1 / 84 .

The scale factor is not in inches.  It's a factor, and has no units.
A 1-ft model would represent a real 84 feet.
A 1-meter model would represent a real 84 meters.
A 1-yard model would represent a real 84 yards.