I seem to have a collection of remote HO code 100 switches (turnouts) which extend straight for about a centimeter past an otherwise matching piece of 18 degree track.
Are there any which match exactly?
Most important reason is I like running trains while finalizing my layout. It hold my interest better and gives me new ideas. If I can drop a switch right in to replace a piece of track when an industry gets built up and needs a spur is all the better.
Not that I am above using the cut off tool to shorten some track. I just hate opening a new switch package and cutting immediately lol.
The standard geometry from Atlas and the copies for Snap Switches is a 9" straight, and a 20 degree curved section of 18" radius. The curved section starts 1.5" from the end of the turnout to provide physical protection and alignment for the points and stock rails. If you cut off the full 1.5 inches, the switch will be much more fragile. And derailments will be much more likely unless you take extra care to avoid any kinks or bumps or dips at the joint by the points. Also, you need some straight left to put on rail joiners without interfering with the points - unless you don’t use rail joiners.
Cutting the straight part down to about 1/2" or slightly less is practical, but does put a premium on laying track without kinks, bumps, or dips. But when you do that, the straight part is no longer a drop-in for a 9" straight section.
So there is no magic way to make a drop-in replacement for a 30 degree 18" radius curve section. Some alterations will be required. Atlas track plans insert 1.5" straights or a matching turnout on the opposite side of the oval to compensate.
You probably mean 18" radius track. Fred’s advice is correct, the Atlas Snap Switch replaces a curve, but with the addition of 1 1/2" in front of the curve.
Note, though, that for technical reasons (the sharp angle of the frog) the Snap Switch is effectively sharper than your 18" radius curves and will be the limiting factor to the length of equipment you may run. Best to try it with the longest locos and cars you have before laying track permanently to be sure that you aren’t disappointed with the final result.