What is RR?

Actually the correct spelling IS Gage. And, perhaps there is a term "gauge RR" which refers to the size of railroad tracks. But the Six Sigma methodology for process improvement includes a step called Gage R&R. The R&R stands for reliability and repeatability. This is part of the process of verifying your data for analysis - determining if the same results would be obtained regardless of the operator, hence "reliability (accuracy of the data) and repeatability (will it be the same each time, all else being equal).

AnswerIt's the size of the tracks a train uses The spelling you want is GAUGE. This is not a complete answer but you can easily get more through a search.

Gauge of track differs between for railways people use, model trains, and toy trains.

This is not a complete answer.

Another Answer

The first answer is pretty much correct except that Gage R&R does not check accuracy of the gage. This is a separate test. A gage can be repeatable (give pretty much the same answer for one operator) and reproduceable (give pretty much the same answer from operator to operator) but still be giving the wrong size. R&R is a statistically-based number that identifies the probablity of getting the same answer, but not necessary the correct answer.