O Scale - 1:48
Gauge: 1.25"� (31.8mm)
Marklin, the German toy manufacturer who originated O scale around 1900 chose the 1/48th proportion because it was the scale they used for making doll houses. In O scale 1/4 inch equals 1 foot. Interestingly, O scale was originally called Zero Scale, because it was a step down in size from 1 scale. From the 1920s until after World War II, O scale dominated the model train market. But as model trains became more affordable for the average person, the space required to set up the tracks became a major consideration in purchasing model railroad trains.
HO Scale 1:87.1
Gauge: 0.649"� (16.5mm)
HO has the broadest range of products from the greatest number of manufacturers. In HO scale 3.5mm equals 1 foot. The designation HO stands for "Half O". Of course if it was actually one-half of O scale it would be 1/96th, but because HO is actually a metric based scale and O is based on English units of measure the fractions become awkward. The model railroading industry says HO is "approximately half O".
S Scale - 1:64
Gauge: 0.884"� (22.4mm)
S scale trains were introduced to the model railroad market by A.C. Gilbert as American Flyer products in 1939. In S scale 3/16ths of an inch equals 1 foot. Though very few companies manufacture S scale trains today, the American Flyer brand still exists as Lionel product line.
TT Scale - 1:120
Although invented in America after World War II, this scale is popular in Russia, eastern Germany, and other countries of the former Soviet Union. The use of 1/120th scale is common in engineering diagrams because it allows 1 inch to equal 10 feet. Though TT scale nicely fills the niche between HO and N scale, it hasn't received great acceptance outside of eastern Europe.
N Scale - 1:160
Gauge: 0.353�" (8.97mm)
The "N" is short for nine millimeter (although the actual gauge is 8.97mm). In N scale 2mm is approximately 1 foot. N scale is the second most popular scale worldwide. Many modelers select N scale as an alternative to HO scale because it allows more complex layouts to be built in the space available to them. Traditional thinking is that N scale trades detail for space. However, modern manufacturing and painting processes are producing N scale models with surprising levels of detail today.
Z Scale - 1:220
Gauge: 0.257"� (6.52mm)
Prior to 2008, Z scale was the smallest commercially available scale in model railroading. Z scale is popular with apartment dwellers and others with very limited layout space. Z scale is awkward mathematically, approximately 0.0545 inches to the foot, but that doesn't hurt its popularity. Z scale is the fastest growing scale in model railroading today. Some Z scale manufacturers offer briefcase layouts; entire track layouts with landscape, buildings, and a power supply all in a briefcase. These are high-end executive toys that sell for around $1,000.
OO Scale - 1:76.2
In OO scale 4mm equals 1 foot. This metric based scale is the most popular scale in the United Kingdom. The British firm Hornby is the largest manufacturer of OO scale trains.
1 Scale (One Scale) - 1:32
Gauge: 1.75"� (45mm)
The numbered designation "1 Scale" dates back to before the turn of the 20th century. Although nearly forgotten after World War II, #1 scale regained some popularity in the UK as far back as the 1960s, and is experiencing a renaissance today in North America. The major model train manufacturer MTH is one of the new sources for modern #1 scale trains in North American prototypes.
T Scale = 1:450
In January of 2008, KK Eishindo Co., Ltd. of Japan introduced T scale. The "T" stands for three millimeters, which is the gauge of the rails. Initial offerings include West Japan Rwy. Co. trains in 4 color schemes and East Japan Rwy. Co. trains in 5 color schemes.