Train Scales explained - The scale of a model is its size is in relation to it real life size. The sizes are listed below from largest to smallest. The Gauge is the distance between the rails of the track in the model train. The larger the Gauge the wider the track. The larger the scale the taller and larger the train.

G Scale Trains- G Gauge

Brands include Piko, LGB, Bachmann, Aristocraft
G Scale (often referred to as Large Scale) would include a Garden Railway or G Scale is great for the Larger sized Christmas Tree. G scale Train sets typically come with enough track in the starter sets to complete a 4 x 4 foot circle. Due to its Size, G scale (45 mm or 1 34 inches, G gauge) is a track gauge for model railways which, because of its size and durability, is often used outdoors. Piko and LGB Tracks are made of brass rails which are great for outdoors because they will not rust. The Gauge or width of the track is 1.75" between rails. See G scales Trains

O Scale (O gauge) Trains

Major Brands include: Lionel, MTH Trains, Atlas.
If using O Scale for Christmas layouts, O Scale Train sets fit comfortably around an average  med- large Christmas tree.
The Scale is 1/48, and it's an extremely popular size. The train starter sets in O Scale typically come with track that completes a 4x8 oval. O27 track was used in the 1927 when Lionel purchased Ives Trains. Eight Pieces of O27 makes a 27" circle. O27 gauge has a lower height than O ( very slight). Around 2016, Lionel stopped producing the O27 gauge track and only makes the Fastrack in O Gauge track.
O27 gauge trains will run on the O Gauge track. SOME O gauge trains will NOT run on the O27 Gauge track as the curves are too tight. The tightest curve in O Gauge is the O-31 radius and there was a 27" radius in the O27 gauge track. Fastrack is the current track made by Lionel and is O gauge. Keep in mind that the gauge has nothing to do with the size of the curves. Curve Sizes currently made in O Gauge Fastrack is 31",36", 48", 60", 72",84", 96" in Diameter. The size of the curve required will depend on the engine. Each engine has specific requirements for the minimum curve. The Gauge or width of the track is 1.25" between Outside rails (often as in Fastrack the track will have 3 rails).
See O scale Trains Sets

American Flyer Trains

are slightly smaller than the O scale trains and only made by Lionel.
There are only a few train set currently in American Flyer including the Lionel American Flyer Polar Express Train Set which comes with track to complete a 50" x 73.2" oval. (Not to be confused with the O scale and HO Scale Lionel Polar Express Train Set). American Flyer only had 2 radius': 20" (40" diameter) and 27" radius (54" diameter). American Flyer is also called Fastrack but it is S-Gauge Fastrack and has 2 rails (o scale has 3 rails) The Gauge or width of the track is .875" between rails.
See American Flyer Trains

HO Scale Train Sets

(half O Scale size which is where HO derives it name!) Brands include: Bachmann Trains, Lionel trains, MTH trains, Atlas, Athearn
HO Trains are 1/87 scale. These trains would be great for a smaller christmas tree. The HO Christmas Trains usually are packaged with track to complete a 47" x 38" oval. This Size is too small for children and NOT recommended for tiny hand or children under the age of 14. Since HO is Half the size of O scale, a city layout would take up a smaller space than the O scale train layout. Most HO trains do Not have the sounds and smoke that you will see in the O Scale trains. For Realistic train sound you will want to look at O Scale. If you live in an Apartment or small space and do not have small children, HO scale is perfect for you. The Gauge or width of the track is .625" between rails.
See HO Trains sets

N Scale Train sets

Small in size (but not the smallest!) N Scale (1:160 Scale). We We do not currently stock N Scale trains but will special order them. The Gauge or width of the track is .375" between rails.

Z Scale Train sets

Very Cute tiny size. Z (1:220 Scale). We only carry z scale during the holidays and it is meant for a table top display. Complete Track is 5-3/4 x 20" oval. One of the nicest z scale layouts I have seen was an entire village that was displayed inside a child's small guitar case! Now that is imagination! The Gauge or width of the track is .25" between rails.