A very small wheel puller is used to pull the flywheels and gears off, such as part # HH-1032 here.
Getting them on again properly is a different ballgame. The simplest way is to use a vise with proper spacers. A good vise is needed here, because cheap ones do not close with the jaws perfectly parallel, so there is a chance of pressing the parts on crooked. Ideally, a small arbor press is what is needed to insure a straight press. A Lionel repair shop could possibly use their wheel press to accomplish the job for you.
Larry