Rule 1: 1. The relative pronoun before a verb
A relative clause very often consists of a verb: اَلْقِطَارُ الَّذِي سَافَرَ the train that travelled. When the noun described is the object of that verb, Arabic puts a pronoun suffix on the verb pointing back to it: اَلْقِطَارُ الَّذِي رَكِبْنَاهُ the train that we boarded (literally that we boarded it). After a preposition the same returning pronoun is needed: اَلْقِطَارُ الَّذِي تَسْأَلُ عَنْهُ the train you are asking about.
اَلْقِطَارُ الَّذِي يَذْهَبُ إِلَى الْقَاهِرَةِ سَرِيعٌ.
al-qiṭāru-lladhī yadhhabu ila-l-qāhirati sarīʿun
The train that goes to Cairo is fast.
The noun is the subject of the verb: no returning pronoun.
اَلْحَقِيبَةُ الَّتِي حَمَلَهَا أَبِي ثَقِيلَةٌ.
al-ḥaqībatu-llatī ḥamalahā abī thaqīlatun
The bag that my father carried is heavy.
The noun is the object: ـهَا returns to it.
اَلْمُسَافِرُونَ الَّذِينَ تَأَخَّرُوا انْتَظَرُوا قِطَارًا آخَرَ.
al-musāfirūna-lladhīna taʾakhkharu-ntaẓarū qiṭāran ākhara
The travellers who were late waited for another train.
اَلْمَحَطَّةُ الَّتِي وَصَلْنَا إِلَيْهَا قَدِيمَةٌ.
al-maḥaṭṭatu-llatī waṣalnā ilayhā qadīmatun
The station we arrived at is old.
Returning pronoun after the preposition إِلَى.