I have called them switches my whole life ("O22 Switches", remote control switches, O27 switches, manual switches, etc.) but whenever I read "turnout", I know what is meant and I translate it to "switch" without giving it much thought.
I recently purchased a dandy of a letter from 1878 written by the Superinendent (President) of a railroad in which both the word "switch" and "turnout" are used in the same letter. The letter is one of the artifacts I collect from the Providence, Warren & Bristol railroad. The PW&B RR once served my town here in Rhode Island.
The letter, shown below, was handwritten by the railroad Superintendent to a lawyer. Apparently, men working for (or represented by) the lawyer had left a switch "set for the turnout" position on a Saturday night and it was "only by the care of [the railroad's] men that a serious accident was avoided".
I own or have read many letters by this particular RR leader and I can tell you that he was boiling with rage when he wrote it. It's a clear departure from his customary business-like prose.
It's pretty clear that, since the letter is from 1878 the use of the word "turnout" dates from quite some time ago. I could be reading it incorrectly, but at least from this man's usage, it would also seem to indicate that a "switch" is the entire apparatus (i.e., entrance, points, left leg, right leg, and the switch machine) and the "turnout" is the track that turns away from the mainline.
But that might be wrong. I'd be interested in hearing more from others.
Steven J. Serenska