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A set of 8 Trixbrix R88 tracks to form a quarter of a circle (90 degree coverage).
The R88 curve radius comes next to R72 when building train layouts with a distance of 8 studs between tracks. Wider trains can safely pass this way and there will be enough space for switch levers, signals, platforms etc. It also makes your layout look better organized and it will keep your layout within absolute geometry. This makes building railroads exciting and fun! 
Dedicated ballast plate: Ballast Plate R88 Set - 8 pieces for 8 R88 tracks
Detailed explanation of R88 radius
The R88 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 R88 circle can be calculated as follows: (88 + 4) * 2 * 8mm = 1472mm = 1.47m. 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