Ah, those classics of fantasy, the enchanted -- things. Some are age-old; magical rings are found in the folklore of every culture that wears rings, and magical swords, every culture that uses swords. Some are newfangled; magical trains sprung up only after trains.
They require some care in handling to be more than transparent plot devices, with two main points.
The first point is that you want the object to be either simple or subtle.
If it's simple, it predictably does one and only one thing -- or perhaps a small group of things, all related -- but acts stupidly. It pours water. It lights fires. It grants flight. It may be very powerful in doing it, in that it could create a lake with its waters, or lift a city with its flight, but it does only its thing. If characters wish to use the power outside its normal arena, where it's not stupidly obvious how to apply it, they must devise a crafty way to use it. Crafty ways are the best to use them. Simple and obvious applications can have only a minor role in solving your story's problems.
(The object also can carry the fearful danger that it's going to go haywire, like the mill because of which the sea is salt. If such a threat is useful to move the plot, of course.)
If the effect is small enough -- the ring that lets your hero light a fire with ease -- it can pass more or less unnoticed as part of the background of the story. Still, even the most trivial of enchanted objects gains if its use, at least some of the time, requires the use of the characters' wits. And definitely if it is the plot device that saves the day.
Subtle objects, on the other hand, can produce a wide variety of effects, but for a good story object, the effects are not easy to reproduce. If, indeed, the effects are not driven by the object's having a mind of its own.
What I disliked about Mercedes Lackey's sword Need is that it gives the woman who wields it what she needs. This is never, ever, in any story, a puzzle to her. Of course, such a puzzle would need to happen early in the story, with anger and bafflement from the characters so the readers don't think it will slide, and then in the end, have an explanation to prove it wasn't arbitrary. Indeed, it probably has to be wise, to work. Given that its motives can only be gleaned through its actions, a foolish decision would smack too much of the author's decision.
In comparison, Wildcard, from Tales of the Questor, was justly named for its wild and random effects. When he actually needed something, what it produced turned out, in the end, to be useful; face to face with cruel and blood-thirsty hunters, it produced basically a pink garland more suitable for a festivity, making the hunters laugh and gloat for some nerve wracking moments -- that proved to buy some crucial time for our hero. When he does not need it, when he just tries to test it on a practice field, once the sword announced that at the sound of the tone, it would be exactly a given time, and then it got the time wrong.
Then, this all can be finessed if the object can talk. If it can talk, it just becomes another character, and its actions are governed by its personality and purposes, as a character. Its limited ability to act has been a stumbling block to many a writer. Most intelligent and vocal magical objects I have seen have been annoying. It may be wise to make the object wise just to avoid that.
The best magic item I ever read of was in Piers Anthony's Castle Roogna. A ring (vocal under the influence of outside magic) claimed to be a wishing ring. When you made a wish, it claimed to be working on it. So you went and did the work to get what you wanted yourself -- and it claimed credit. But -- every wish made on that ring came true, sooner or later. That's the sort of quirk that makes a magic item really worth creating.
But this is only the first point to consider.
The second point is that you need a metaphorical congruity between the object and the purpose. When unlocking things, use a key, not a sword, or a cup. Swords are best for damaging things. The sword that cuts anything, or just about; that inflicts wounds that do not heal; that requires blood to be resheathed, magic not needing to be nice -- all of these fit swordly nature. Invisibility cloaks envelop you. Boots travel and are suitable for any such magic, from seven league boots to ones that allow you to move with stealth. Rings are subtler, because you can metaphorically put on, or take off, the power they grant.
Except for comic effect, of course. Incongruity between purpose and form is a great source of comedy. Or possibly because the incongruity is insane, to underscore the metaphysical discord of a setting.
One reason why magical trains are much more frequent than magical cars is that there has been more time to make them familiar in stories. But the more profound reason is that cars do not offer any symbolic significance that chariots, flying carpets, winged horses, hippogriffs, and all the rest already have. Railroads, on the other hand, are bound to a fixed route to an extent that no other form of transportation is, on top of being driven by the crew with no input from the passengers. Consequently, they are more fitting for fixed destinations -- such as Death -- than even sailing ships, carriages, or buses.
A properly developed magic object will, in the end, fit so well into the story that it will seem as inevitable.
You've written an interesting piece about the use of magical objects!