Rail switches (also known as turnouts and points) are essential parts of any model train layout. Rail switches are the connection points that allow a train to move between different rail paths. Whether it's a single switch to a small industrial siding, a major junction between two lines, or a busy yard ladder, switches make model railroad operations possible.
The Switch Classification System
Railroads (both real-life and models) use a numbering system to classify the frog of the switch—the point at which the left rail from one track crosses the right rail for the other track. As defined by the North American Rail Products glossery, the frog is "a track structure used at the intersection of two running rails to provide support for wheels and passageways for their flanges, thus permitting wheels on either rail to cross the other."
In the standard classification system, the larger the assigned number, the longer the frog. Modelers commonly use that same frog number to classify the entire switch. The number assigned to the switch (or the frog) is determined by the units of lateral track run required to create a separation of 1 unit between the centerlines of the two tracks. In modeling, the designated measurement unit is usually inches. For example, ff it takes 6 inches of track length for the center lines of the two tracks to get one inch apart, you have a #6 switch. Note that this doesn't mean that a No. 6 switch is 6 inches long; generally, a No. 6 switch will be about twice that length when measured from end to end.
One common misconception about model train switches is that a classification number corresponds to a particular radius of the track. Switch geometry doesn't work this way, and switches aren't configured in a constant curve radius. There are exceptions to this; some common switches are sold as replacements for a standard rail curve piece—an 18-inch radius for example. These switches look very close in size to a No. 4, but they are not the same. Most O-Gauge 3-Rail tracks are also marketed this way.
Unlike a typical "left" or "right" switch, wye switches have tracks that diverge in two directions simultaneously. Because of this, the numbering of a wye can be misleading. A No. 3 wye switch sounds like the angle of divergence would be extremely sharp, but it actually matches a No. 6 standard switch.
Wye switches get their name from their shape, and also from the track arrangement of the same name. A wye is a rail configuration used to turn locomotives or trains. It can be built with or without a wye switch, and wye switches can also be used in other applications.
On curved switches, both tracks curve in the same direction, with the inner track forming a sharper radius. Some manufacturers do market them this way; other manufacturers use numbers, while others simply say "large radius, medium radius, etc."
All of this diversity can make things very difficult when planning a rail layout, even for experienced modelers. Some manufacturers offer online templates that you can use to plan the rail layout, and there are also track-planning software packages that are programmed with accurate templates. In other cases, you'll have to make your template from an actual switch.
Despite those efforts, often your only options are to buy a curved switch that looks close and work around it, or build your own custom-fit switch for your situation. You can build your own switches by hand, or from kits. Building a switch is more work than simply tacking down a track piece, but the resulting look and operation of a switch that flows with the track is worth it. As with any skill, the work will go much faster after you've completed a few switches. Given the higher cost and limited options for curved turnouts, this is the best option for most people.
In addition to switches where one track diverges into two, there are a number of special switches with multiple exit points. These are sometimes collectively known as "puzzle switches," and can often be found in busy junctions or terminals.
A three-way switch is a switch in which a single track divides into three diverging routes—essentially a left and a right switch combined. These can save a lot of space in compact switching areas.
Single-slip switches and double-slip switches involve a pair of tracks that cross at an X-shaped diamond pattern (this switch is sometimes called a diamond crossing). In the single-slip version, the train has only one option as it crosses the other set of tracks; in the double-slip, it can choose from either set of tracks as it crosses. These switches are most commonly seen in large passenger terminals where multiple arrival tracks fan out into the station platforms.