Not all "loops" are loops. A loop properly so called is like Tehachapi and Williams - the line crosses over itself. The Georgetown Loop in Colorado is another one, and when the Roaring Camp tourist line near Santa Cruz was built it had a loop (later becoming a switchback after the bridge burned). There are (and were) not a lot of them. But a turn through more than 180 degrees is also sometimes called a loop, as in the Cantara Loop on the UP between Dunsmuir and Mount Shasta. Would be interesting if somebody knows of more "real" loops, as well as those that don't actually cross over but are called one.