What you are looking for in a motor for an animation project is typically called a low rpm (revolutions per minute) "gearhead" motor. A gearhead motor has reducing gears built into the motor, but some have separate gear assemblies. Usually a motor 5 to 30 rpm is useful for model and toy train layout projects. Another desirable attribute would be for the motor to operate on low voltage, under 24 volts, either alternating current (AC) or direct current (DC). A DC motor is often reversible by reversing polarity and you can fine tune the rpms by how much voltage you give it. But that's not all. Quietness is also important, and lastly, price. I have used "timing style" motors that operate on full household current, 110-120 VAC, however this is not a good idea for a beginner, and is not for those who have beginners and children using or working on the layout. Many extra precautions need to be taken when you introduce 120 VAC into the layout. These motors need to be installed according to the manufacturer's specifications and are not usually designed for homemade projects. And all electrical connections to 120 VAC motors need to comply with local and national electrical codes, so best to use only low voltage unless you consult with a qualified, licensed electrical engineer, and have that qualified individual do the installation. Although you may see 120 VAC motors used in certain projects on this site, use only low voltage motors, properly installed.
My favorite, quiet, reliable motor is by Hankscraft, and you can find Hankscraft motors by searching Google or check Edmund Scientific. You may have to buy them in a certain dollar amount or quantity, but contact the various distributors for instructions on ordering. My favorite source in general for all my hobby electronics is All Electronics. They often will have various styles of motors, but the ones that are most useful for model and toy trains often sell out very quickly. Connecting a turntable or mechanism to these motors can often be done with an epoxy or super glue, provided the shaft has a "flatted" surface, either as manufactured or filed by hand.