Traffic calming has another reason, it makes the street more accessible to pedestrians. They build speed bumps where there is already pedestrian traffic, and they require some assistance. The car equivalent of this would be a speed bump on a high speed motorway, it’s only reason is to annoy some users, without really benefiting anyone else. Or building a roundabout with only 2 exits.
one person occupies it for a long period of time
You just said “a homeless person” with different words. Remember, this is at a train stop. Everyone will leave with the next train. But they add things like this on benches in parks as well, where not just the homeless would like to lie down. But they sacrifice the comfort of other users just to mess up with the unlucky.
for several people to use when needed for shorter periods.
Yes, that’s a legit reason, but the problem is not just this. Why this thing exists at all? Why would anyone want to sleep on a bench on a train station? Because they can’t find a better place to sleep. So there is a much more complex social problem behind the scenes, and solutions exist to mitigate it to an acceptable level, but they are expensive and won’t get you votes, so nobody cares.
This armrest is an epitome of not dealing with the problem just making it look like it doesn’t exist, because homelesses now sleep somewhere else. And you just payed for something which is not a solution, but moves the problem from one part of the city to the other part. What will happen if all benches and horizontal places will get some anti homeless details? Homeless will magically disappear from the world?