It has to be really huge or spin really fast? Sorry, my physics knowledge evaporated.
Sadly, they do rather require "magical" technology....
From a radius of ~300m onward you'll see green lights (= good for people) for all considered parameters. RPM drops from 1.7 to 0.5 for a radius of 3000m.
If you made something like a gravitron ride on the moon, would it take a slower rotation speed to reach perceived 1g than if you spun up a ring station in orbit? This calculator makes it seem like you could get pretty close to 1g with just a bullet train running on a 3.14 kilometer loop.
It seems like the main thing stopping earth trains from being faster is that most of our tracks were built a really long time ago and it's not worth the effort replacing them, but if metal is readily available and you're laying new track already, designing for ~300km/hr wouldn't be that much of a stretch no?
The main thing stopping earth trains from being faster is politics, not engineering. Trains have been occasionally going over 300 km/h since 01955, decades before maglev. The Shanghai Maglev Train has been running at 430 km/hr since 02004. The Euroduplex regularly runs 320 km/hr on regular 1435mm standard-gauge rails and reached almost 575 km/hr in a test in 02007. 300 km/hr trains have been in regular service since 01989. There are several other train lines that run over 300 km/hr, in Taiwan, PRC, France, Belgium, Saudi Arabia, Japan, Germany, the UK, the Netherlands, Italy, Spain, Korea, and Switzerland. Soon India and the US will join them.
The big advantage of maglev is actually not smoothness or absolute speed but acceleration and deceleration.
https://en.wikipedia.org/wiki/Euroduplex https://en.wikipedia.org/wiki/High-speed_rail#Speed https://en.wikipedia.org/wiki/List_of_high-speed_trains https://en.wikipedia.org/wiki/Maglev#Comparison_with_convent... https://en.wikipedia.org/wiki/Taiwan_High_Speed_Rail https://en.wikipedia.org/wiki/Bombardier_Zefiro https://en.wikipedia.org/wiki/Centripetal_force https://en.wikipedia.org/wiki/Euclidean_vector#Addition_and_...
Being huge means less inner ear dizziness, spinning small and fast is stressful on materials and makes humans sick. And docking would be a major POA.
If we are talking km diameter structures holding station in system, making it rotate isn’t going to be a drama.
Other challenges include how to spin it up (and down) safely, how to dock with non-spinning things, how to deal with changes in mass distribution, and how to put thrusters on it for use when it's spinning. None of these is impossible, but together they create a serious engineering problem, and the size of the whole thing is ultimately the dealbreaker.
Docking would be via a central hub. Ships would have to match rotation to dock, but it shouldn't be too hard. My conclusion is that if the money and/or political will were there we could start doing this today, but the project would be hugely expensive (even with SpaceX cutting launch costs to a fraction of what they were only a few short years ago) and once you have it built it will be looking for a purpose. It would be cool for people to basically commute up to the central part (via elevator) to do zero-g research stuff, then commute back to the ring to live and avoid the various health problems with long term zero-g living like bone density loss.
You can even build a simple starter station that has only two segments on opposite sides of the central hub. This is less cool since you don't get the jogging path around the station. If stability is an issue you could also include a computer controlled mobile counterweight on the ends. I also had the concept of building it as a double hull with a layer of water between the inner and outer to reduce radiation flux and absorb micrometeorite impacts.
But in the end you are still talking about a hugely expensive project that solves problems that aren't all that bad yet. About the only way I could see this being built is if Elon decides to go all in on space and liquidates his fortune to build it. The instant some annoying bean counters ask the question "is this the best way to spend this money" the project is dead.