I'm new here. What exactly does an O-42 switch mean?

Yes the curved part would be an O42 curve. Welcome to the forum!!

Bill

And note also that a Lionel 042 switch, the manual version of the 022, has a 45º 14.14" radius(center rail) turnout leg for use with matching standard O-31 curves.

You'd be surprised how often that confusion comes up.

An 0-42 switch is a handle or button that actuates the movement of the points on an 0-42 turnout.

You may see variation in how diameter is measured in O. Sometimes (early L) it refers to the outer dia.; not the track center as you might be used to.  I.e., check each beforehand, an Atlas 042 and and early Lionel 042 may not line up proper.  But cutting/blending is possible.  Don't let pins/connectors be a holdup.  If butted end to end, shimmed for height, and both tracks are screwed down; the wheels wont care much about missing pins and can transition between two track types.

Many turnouts in O are curved vs say a #6, which is normally not curved, but represents a deviation angle.

The curves in the turnouts usually end at 45°, 33.33°, 22.5°, etc, etc. and take respectively 4,6,8,12, etc etc, pieces to make a full circle.   So , yes a curved 042 portion of the turnout takes the place of one piece of 042 track if both are of the same general design.

Where 8 (45°) is needed for an O; to add a straight stub siding on one turnout; now you only need 7 curves,+ 1 (45°) turnout, +a straight traçk to make the O into a Q.

The Fastrack design actually has a small addition to curve to cover two exit angle possibilities.

@Joe_1971 posted:

Hi, This is my first post on the forum.  I've been modeling in HO and On30 for 30 years so model railroading is nothing new to me.  Last year I decided to get started in O scale/gauge.  So, I know that an O-42 curved piece of track would be used to make a (more or less) 42 inch diameter circle.  Here's my question, if I have for example a couple of Gargraves O-42 switches (left and right) would the curved part of the switches form a 42 inch diameter or would the straight part of the switches or both?

Thanks,

Joe

New blood is always nice. On30...very appealing.

Isn't it weird that all other model RR formats (that I know about, anyway) refer to curve radius, yet we here in 3RO - from the toy train end to the extreme Hi-Rail bunch - use diameter?

Actually, it's an old subject and due to the never-died toy train legacy of 3RO, but every time I read a 2RO article, I'm always having to remember to multiply by 2 when they mention curve size.