Railway cars marked S.A.R

Why are the railway cars during Winston escape marked S.A.R ? Presumably for Suid-Afrikaanse republiek, the official name of Transvaal.

However, Dutch (and not Afrikaans) was the official language of Transvaal and I believe the cars should have been marked Z.A.R (Zuid-Afrikaansche Republiek). Unless the S.A.R. means something else of course.