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The R104 radius curve comes next to the R88 curve. It maintains 8 studs between the tracks and enables you to easily connect a wide variety of Trixbrix R104 switches, crossovers and crossings. 
Detailed explanation of R104 radius
The R104 is the radius measured from the center of the circle to the centerline between the rails using the LEGO® stud as the unit. LEGO® stud is 8mm. The space needed to have the full R104 circle can be calculated as follows: (104 + 4) * 2 * 8mm = 1728mm = 1.73m. This measures the diameter (2 * radius) of the outer shape of the track - that's why we added 4 studs in the formula.
In general, the formula for calculating the outer diameter of the track circle is:
(R + 4) * 2 * 8mm