I believe it is 48 in centre line so 51 in to end of sleepers sounds plausible....
the mentioned numbers are near enough.
i know for sure, that the ties are 86mm long. (3.38" )
so from centerline to the outside there should be a mere 1.69".
from the horse's mouth:
Das LGB-Gleissystem | The LGB track system
Going in Circles
Radius Diameter Diameter:
Name Center-to-Center - Outer Edge-to-Outer Edge
R1 1200 mm - 1286 mm ( 47.24" - 50.63" )
R2 1560 mm 1646 mm
R3 2390 mm 2476 mm
R5 4640 mm 4726 mm