BRIO sells a battery powered steaming train, numbered "H2O" on the side, that produces little puffs of steam. It's very cute, but regularly stops producing steam, or the steam is very weak.
The steam is cleverly generated by a little ultrasonic humidifier that's in the main chimney, but ours seems to clog up after a while, probably limescale or some other mineral from the super hard water we have. Our solution is to use a cotton buds (cotton swabs/Q-Tips for Americans) with a little bit of something acidic on it, like vinegar, or vinegar essence, to very gently wipe and clean the metal plate in the chimney. Once cleaned, and given a minute to start puffing again the smoke looks much better. Happy Steaming.