Trains from Copenhagen to Kalmar, Sweden

Train travel is mass transit, it is not at all like air travel, so do not expect to have an air travel like experience when booking air travel.

Suppose you needed a ticket from Penn Station in New York to somewhere on Long Island. How long in advance would you buy that? And where would you expect to buy it?

The trains on that route are operate by Öresundståg which is a commuter railway connecting Southern Sweden to Copenhagen.
https://www.oresundstag.se/en/about-oresundstag/where-do-oresundstag-operate

The vast majority of people traveling on that system either have a commuter pass, or have bought tickets right before travel from a ticket vending machine, or using their phone. On boka.oresundstag.se you will see that trains run every hour, and that the price is fixed. If you look at May you indeed get funny results. Usually when you get unexpected results when looking up train times that means that you are far to early and you are looking at incomplete schedules.

Just do what everybody there does: Go to the station, buy your ticker, hop on a train and go.

Seat61 has a bit of info about what those trains are like:
https://www.seat61.com/trains-and-routes/oresund-train.htm

Thank you for the very fast reply. I assume this also applies to trains within Sweden?

You mean trains?

Long distance high speed trains have yield managed fares, so there you can often get discounts if you book in advance. And it is common for reservations to be mandatory.
But for regional trains you can just buy a ticket and hop on.

What are the other trips you plan?

We also plan to take a train from Kalmar to Stockholm. We’d like to take the scenic route recommended by Rick via Linköping.

That is also just a normal commuter train, fixed fares, no need to book.

Thank you!

Öresundståg are a bit late with ticket release. They don't offer seat reservations any longer and sell an unlimited number of tickets so there is no reason to buy in advance.

If you use the Skånetrafiken app or their ticket machines at Copenhagen central (I think they are close to the exit to platform 1/2) you avoid the booking fee and also get a group discount starting at two people (total for 2 is 790 SEK instead of 1066 SEK). You can't buy in advance from the ticket machines, in the app you can buy first and activate the ticket later, just before you board, but you need to be careful when answering the activation question after purchase.

Thank you, very helpful