Yet another frustrating example to use ! About 4 questions earlier this was the question:
A gyerekek felszállnak a hosszú vonatra. The children are getting onto the long train. FELSZÁLLNAK was used for "getting"
In this example, FELLÜLÖK was used for "get"
In previous lessons (preverbs) FELÜLÖK was always used for "sitting up"
It's these kind of things that make beginners want to give up learning Hungarian.
I try to offer a maybe confusing explanation:
Én felülök a lóra.
I mount the horse. (at least i hope this does work in Hungarian too, since German would use literal "belép" (einsteigen/zusteigen) or "beül" (hineinsetzen) for trains since we travel in trains and not on trains, but we would "aufsitzen" felül on horses. Maybe there is another verb for mounting in Hungarian. I am guessing here! But the important bit is anyway to focus on mount for a bit!)
Mount is a special verb for get on something, right? An animal or bike. It's not only one motion. It describes the get on part, especially that is of interest in present tense, but also the usually following sit part, which is in the past tense more prominent.
I mount the horse. I get on it.
I mounted the horse. I sit now on it.
And felülök is similar here. You are not sitting and standing up.
We just change horse to train: You are getting on the train, followed usually by seating yourself, or stand if no seats are free.