What is TT scale?
TT-Scale ( TT=Table Top ) is half way in size between OO-HO and N scales.
TT track has a gap of 12mm between the rails.  This is called 12mm gauge.
TT scale is 1:120 of the original.  1:120 means 1:10th inch = 1 foot  and  one inch = ten feet. 
So TT scale is easy to convert to real size ..... in TT scale a 40 foot container is 4 inches.
A 50 foot railway coach would be 5 inches ... a 6 foot person is 6/10th inch and so on.

TT is a good choice for layouts with a space requirement smaller than HO, but bigger than N. 
When TT was introduced, it was the smallest model railway scale available. For this reason alone it was extremely popular when it came out. 
Nowadays other smaller sizes are available, and TT is the "medium size".
Because TT is mainly a European scale, most TT parts are made in Europe where quality standards are high.
German precision manufacturing and quality parts are supplied in train sets.
Choosing TT scale for your model trains results in reliable running trains.
Many model railroaders recognize that TT is the smallest practical scale taking into account the size of a modern house. 
TT scale is 3/4 the size of HO, and it takes under 2/3 of the layout space.

Elite Track from Tillig comes in both HO and TT size.  Elite track is one of the best quality tracks available, with weathered brown rails, real height rails, and authentic sleepers and junctions.  Even if you did not choose TT scale, and went for the bigger HO instead, you should still consider using Tillig Elite track in the HO size. Your trains will run better, with fewer derailments, and most of all - your track will look better.

 In the UK  TT scale is called "3mm scale" (3mm = 1 foot).  There is a "3mm Society" in the UK.  This alternative name for TT  is not helpful to beginners who don't know that it means the same as "TT".  So some English and Irish modellers may not have heard of TT.  
In Europe it was always called "TT scale".
When N scale appeared at a smaller size,  it reduced TT market share in the UK and US for several years. However in Europe TT maintained it's place  as second after HO.  TT is very popular in Eastern Europe, and also in the Russian Federation.  It's recent popularity surge in Western Europe, Britain and Ireland started with German reunification.
Eastern European countries like East Germany, Poland, Czech Republic, Hungary etc are now EU members and we have excellent access to TT trains and parts.
Now that information about TT scale model trains is freely available in Ireland and UK the advantages of this system are better appreciated.. 
Many European modellers moved to Ireland in recent years and brought their European model rail knowledge with them.

TT modellers now come together in TT scale Internet modelling discussion groups.  So UK and Irish modellers see more TT scale than ever before. Green Hobby & Model in Dublin have produced English versions of a lot of the German language TT information, on their website, there is more English language internet information about this scale than ever before. This helps new railway modellers to see the advantages of TT against other model rail scales.

Who makes TT scale model railways?
First Rokal started manufacturing TT in 1949, and by 1952 they had a complete range of locos, coaches, track, etc. 
Later in the 1970s Triang  in the UK  manufactured TTR ( table top rail ), called "3mm scale" in UK.  Other manufacturers in Germany and other countries started making 12mm gauge rolling stock.  Next Pilz developed excellent 12mm track. TT scale landscaping, and add-on parts are also available form many sources,  TT scale in Europe is second to HO scale, and is having a resurgence of popularity. Several small US based makers designed TT items specially for US railways. After German reunification Pilz and several other old East German TT making companies merged into Tillig. Since then Tillig is the biggest global TT manufacturer.

What is available in TT scale?
A huge range of model trains, locos, components, and parts is available to help you make your layout.
TT Track is readily available and is a very high quality product.  Tillig  has the most complete line of track and straight turnouts, curved turnouts and double-slip switches.  The Elite three rail track is an elegant way to have standard gauge and narrow gauge models working on the same layout. These are available from Green Hobby & Model in Ireland who have an extensive TT web site at www.greenhobbymodel.com. 
Other big HO scale manufactures like Roco produce TT scale engines, cars, buildings, and other accessories.  These items can be found through searches on the Internet.    Fleischmann do a beautiful TT turntable. Noch do excellent TT size people, tunnels, and scenic landscaping materials. Other manufacturers such as PMT, Kuhn, and  Sachsenmodell manufacture TT scale locos, Auhagen make bridges, buildings and stations.  A Yahoo or Google search under TT scale will find much of interest. 
Finally .. you can make scale crossovers ... due to the close similarity between OO/HO and TT - a medium HO tree makes a nice medium-large TT tree. A HO factory building is just a bigger TT factory. Same with many buildings, bridges, etc. So you can incorporate many items for these other sizes into your railway layout too. There are enormous possibilities.

What is the minimum turning radius for TT scale?
The "1st" smallest radius is 267mm = 10.5 inches. But that might limit rolling stock choices to "short" goods wagons.
The "2nd" wider radius is 310mm = 12.2 inches  This suits all. 
The "3rd" wider radius is 353mm = 13.9 inches   This suits all.  
You can make wider sweeping turns with flexi track if you desire.
So a "twin oval track layout" with two rail lines can fit into a 2 x 14" width plus a bit spare = 29" width ! 
If the inside curve is 1st radius reserved for goods trains, fast trains can use a 2nd radius outside curve - and width reduces to 26".
So now you can see why it's called Table Top Scale?  Note - When it gets up and off the floor it is safer from breakage. Also -  the need for crouching over a floor based track is eliminated.

Is TT scale more expensive than other scales?
TT scale trains and kits are priced about the same as HO and N scale items.  Some items may be less, some more.  However older and original TT scale items may be more expensive, as some are starting to view them as collectable. 
There is one way TT can end up more expensive .... because TT fits into a given layout space better, you may decide to acquire more "features" and possibly also decide to run longer trains with more coaches.  If you do, your enhancements will add to the cost of your (improved) layout ! (This also applies to N gauge layouts by the way)

What about using digital (DCC) control with TT?
Traditional modellers used standard analogue (DC) systems to control their TT scale trains.  Digital introduces possibilities like sound, individual train control, and simpler track wiring in layouts. The Tillig digital system is made by the highly respected Lenz company in Germany. Roco also have a Lenz developed DCC system, in fact the international standards for digital control were set by Lenz. TT scale has the advantage (like HO) of being large enough to be able to install sound decoders in the locomotives which increases the attractiveness of those locos substantially.

I had OO several years ago - Why should I change to TT scale?
TT scale is larger than N scale and smaller than HO, so it may be an ideal scale for you. 
Compared to OO or HO : - TT allows longer trains, for example most UK made OO Sets will have a loco pulling only two to three carraiges (so the train doesn't reach from one end of the layout to the other ).  Slightly shorter TT wagons allow you to make your trains more interesting by adding more rolling stock into your layout, like some realistic looking 8-10 wagon trains. If desired turns can be tighter than HO or OO .
Compared to N or Z : - N and Z are below TT in size.  In N the trains get small - in Z  the trains are tiny.  Landscaping becomes difficult to do well.
Many N & HO scale modellers say that TT happens to be a good compromise in size.  You can see the details in TT without having to get up really close.

When making your layout ....  TT scale is 3/4 the size of HO, and it takes under 2/3 of the layout space.

Is Narrow Gauge also available in TT ?
Yes. Modellers doing TT narrow gauge can model TT for the standard gauge.  In this case 9mm N gauge can be used for narrow gauge track sections.
There is excellent three rail track to link a narrow gauge N gauge track to 12mm TT track allowing both sizes of trains to run on the three rail section and use the same loco sheds, while staying to their own gauge during normal working operation.  

Alternatively -  Standard TT 12mm track is similar to HOm track. Therefore 12mm TT track can be used for the  narrow gauge part of the layout, where the full scale main layout is HO size.  There are lovely Street Trams available to use this system, running on TT 12mm through the streets.
Tillig make this (see picture) three rail track to link 12mm TT track to 16.5mm HO/OO track allowing both sizes of trains to run on the three rail section and use the same loco sheds, while staying to their own gauge during normal working operation.

There is more information on narrow gauge in the street trams & Luas section. The trams could also be run through the street using the same gauge as the mainline, - all on the same track in other words.  Also .... many modellers will use a narrow gauge to send goods trains off the mainline out to a picturesque quarry or mine.