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There is a different railroad track width in different countries.
The most popular is 1435 mm – „European (Stephenson) track”, 60% of all railroad tracks have such a width. European standard of the track formed during initial moments of railway transoprt development. 
While constructing the first locomotive in 1825 famous English engineer George Stephenson used standards of English cartage, including axis of the distanse between wheels – 4 feet and 8,5 inches. Stephenson used this stencil during construction of the railway Liverpool-Manchester which was ready in 1830. This in turn became a stencil model not only in England, but also in continental Europe and North America. 
The second place is occupied by so called „Russian track” which is 1524-1520 mm and which is used in the former countries of USSR, Finland and Mongolia. Also are known „Cape gauge” – 1067 mm which is widespread in Africa, Indonesia, Japan, „Indian track” – 1676 mm in India, Pakistan, Argenitina, „Iberian track” – 1668 mm in Portugal and Spain and some others. 
That’s why it is agreed that in spite of the difference in railway track width, in various countries in the models with the same scale are used the same railtracks.
When compiling the initial standards of railway models, the most popular models in Europe were those with the scale 1:45, thw width of the track - 32 mm. These models have the identification mark «0» (zero) – symbolically standed for the beginning.
Larger scales got the «1» symbol for the 1:32 scale, «2» for the 1:22.5 and so on.
Smaller scales got the more intricate names. So, 1:87 scale is «H0» («Half 0»), 1:120 scale is «TT» («Top of Table») as a scale of the models which can be laid out on the writing desc. 1:160 scale has a «N» symbol («Nine» or «Neun» in ger) because of the railroad track of 9 mm.

Symbols of the main scales and their dimensions in the table:

    Name                      Railroad track width of the prototype, mm                              
        1250 - 1700 
   850 - 1250 
   650 - 800 
    400 - 650 
6.5 - - -
9 6.5 - -
12 9 6.5 -
16.5 12 9 6.5
22.5 16.5 12 9
32 22.5 16.5 12
I or 1  
45 32 22.5 16.5
II or 2   
64 45 32


Railroad with the narrow tracks have different symbols. Thus, the narrow track version of the 650-850 mm at a 1:87 scale is named „H0e“ and uses tracks of 9 mm at the N scale. Tram version of the track 850-1250 mm at a 1:87 scale is named „H0m“ and uses tracks of 12 mm at the TT scale.

At the moment the H0 scale is the most widespread in the world. In Germany ca 74% of all modellers work with this scale, 19% - N, 7% - TT.
In Sweden 67% - H0, 23% - N. In America 60% - H0.
In the former USSR modelling was connected with the East German producers Piko and Vero. This is why the spreading of the scales on the territory of the former USSR reminded of the situation in GDR in 1990. Thus, in GDR 48% were using H0 and 45% preferred TT.
Nowadays on the territory of the former USSR 47% prefer H0 scale and 41% - TT.
It bears mentioning that the prices of the models with the different scales H0 and TT are practically the same, because tiny details demand more precise production.
The preference of the TT scale is also due to the less space needed. With relatively the same quality level TT scale demands app. 1.5 times smaller surface for the model. The N scale is even more smaller, but the models have simlified look.
For comparison can be added that N scale is the most popular in Japan, where it covers about 85% market.
All information is presented on the base of the market research in 2004-2006.
Also, relatively widespread is 1:22.5 scale, which is also known as a „garden” scale (“G”). Rail tracks of this scale are usually mounted in the house garden. These models are characterised by the high accuracy and are used for miniature steam engines. But in our climate with the showers and heavy snows this scale is impractical.
It should be mentioned that in England the most popular scale is 00 or 1:76.2. This scale is also named “4th scale”, cause it developed from the ratio 1ft : 4mm. But this scale also uses tracks with the width of 16,5 mm, the most popular in the world.