Scale, what is this all about? Do I care? How do I tell?What scale is all this stuff?Why should I worry about scale?What's available, how will it affect me?These are all questions you should address, since the supply of models is limited, unlike a scale like HO, where virtually everything is available.Basically, you want to decide if scale is important to you, and how important.(If you are coming from another scale, the following stuff will seem strange, almost sacriligious, but read on)Often things are called "G Scale".... this can be many different scales, that commonly "fit" on 45 mm gauge track.(there are also "scales" that run on O scale track (32mm) for example 16mm scale, which is 16mm to a foot, and is about 1:19 scale and represents about a 2 foot narrow gauge... it really cannot get much more confusing.)So let's stick to "G Scale", running on 45 mm track also often known a "Large Scale" but stick with "G Scale"What scales run on 45mm track?Well, mostly 1:20.3, 1:22, 1:24, 1:29 and 1:32 !!! (These are the common ones!!)If you are into American narrow gauge steam locomotives, you are in luck, since there are a number of good models available, all narrow gauge. Many of them are somewhat expensive, but there are some good inexpensive ones available from Bachmann. The higher priced stuff is from Accucraft, and a few others. The "reasonably priced" stuff is from Bachmann. The 45 mm track works out to the right size for narrow gauge in 1:20.3, pretty much the standard narrow gauge you can get in the US.If you are into standard gauge trains, and exact scale is important, you have the possibility to buy equipment, but your costs are going way up. MTH makes/made a nice line of models in 1:32 at moderate to high prices, but they are about the only "reasonably priced" alternative in 1:32, and also a lot of "liberties" have been taken to allow locomotives navigate "toy like" sharp curves. (This is from the MTH heritage in O scale). MTH is not terribly active in the #1 scale, and no new locomotives in a while. They have very nice sound systems, and a nice (but non-standard) remote control. Recently they have added DCC compatibility although a bit flawed. You cannot buy the locos without the extra electronics. Your only other locomotive and rolling stock choices are much more expensie, Accucraft and Aster and several other high end manufacturers make beautiful models, but these can run into thousands of dollars for a locomotive. Basically be prepared to spend more on cars and locos.If you are into standard gauge, and exact scale is not so important, USA trains, Aristocraft and AML (American Main Line made by Accucraft) make excellent inexpensive models in 1:29. The trucks are a bit narrower than scale, since the track should be theoretically wider for this scale. Most people accept this in exchange for the large selection and good prices. More and more companies are build in this scale, like Gibert LeCroix' GLX Models.The 1:29 stuff came about sort from the "wow" factor. If your layout is on the ground, the 1:29 standard gauge locomotives look much larger than the exact scale 1:32 stuff. There is also more available in this scale than any other.(of course, if you are in narrow gauge, then the 1:20.3 is even larger, more wow!)If you do not worry too much about scale, you can mix various scales, but right next to each other they can look funny to most people.The oddball scales look pretty strange when mixed with other scales.Worth mentioning here is LGB, who pretty much is responsible for the popularity of this part of the hobby. They often have products in a compromise scale in terms of width and height, but shortened to run on their tightest curves. Next to a 1:29 model, they often look short and toylike but wider and taller. This is excellent quality stuff, but if you want scale appearance, this won't be your favorite. They have gone into bankruptcy twice in recent years.My best advice is go to someone's layout, have them put the different scales next to each other, look at the trains and see what floats your boat!Bottom line: run what looks good to you.