Generally these trains run on gauge 1 track that is 45 mm or 1-3/4" between the rails. Most trains are about 4" to 5" wide and about 6" to 8" tall, the length varies depending on the type of equipment. This size for the commonly run scales, generally called "G" scale or "Large Scale". These will be in the range of 1:32 to 1:20 scale. The 1:32 scale represents standard gauge trains (4' 8.5" between the rails) running on this track and the 1:20.3 scale represents narrow gauge trains ( 3' between the rails) on this track. There are several scales in between that are also designed to run on this track. The common ones are; 1:29, 1:24, and 1:22.5; in the 1:24 scale 1/2" equals 1'. Most of these are designed to run outside in all types of weather.
Starter sets are available in a price range around $200 to $500. These are great for small children as they have few, if any, small parts and are very durable. This size makes it easy for the kids to see if it is on the track. They generally include a locomotive, two cars, a 4' circle of track and a power supply-controller. The cars and loco are four wheeled and easy to put on the track. These are available from LGB, Piko, and Bachmann, also great for running around the Christmas tree
Most of these are electrically powered by up to 24 volt DC current from the track or onboard batteries, but can also be live steam powered. Electric powered models can be controlled by regulating the track power or by some means of remote control by radio or DCC through the rails.
There are several manufactures of these trains, but this size also lends itself to scratch building the trains and buildings
Mile High Garden Railway Society
Most of these trains are designed to run outdoors in all types of weather