"The guests descend from aboard the train which goes to Szeged."

Translation: A vendégek arról a vonatról szállnak le, amelyik Szegedre megy.

[RO_4_PM]

"a vendégek szállnak le arról a vonatról amelyik Szegedre megy" was marked wrong. one of the two allowed answers had arról a vonatról before the verb, the other one kept the order i used but only used a vonatról? why no demonstrative there?

[vvsey]

Your version emphasizes the guests. It is the guests who.... . Not the nurses but the guests, for example.
With a different intonation, it could also emphasize "that train". But then it sounds like only guests are ever expected to be on a train. So it would work better with "passengers" then.

I guess it should be accepted anyway.

"A vendégek szállnak le a vonatról, amelyik Szegedre megy".
This one is relatively neutral. It does not emphasize "that one which", it just simply identifies the train as the one that goes to Szeged.

And the above translation:
"A vendégek arról a vonatról szállnak le, amelyik Szegedre megy."
This one clearly emphasizes "that train which". It answers the question: "Which train are the guests getting off? Or whatever the correct question might be.

[RO_4_PM]

So basically if you are not emphasizing the place or object that the relative conjunction refers back to, then you can omit the demonstrative from the first clause?

[vvsey]

I will say a careful "yes", I think so. You got it. Thanks, good one!
Let's see if this statement stands the test of time.