Langs means next to, beside, or alongside.
In this sentence, langs my means next to me.
It is a preposition, so it introduces a phrase showing position or location.
Because my is the form used after a preposition.
Compare:
This is similar to English using me rather than I after a preposition: next to me, not next to I.
Yes. Afrikaans often allows another element to come first for emphasis, but then the verb still stays in the second position in a main clause.
For example:
This still means essentially the same thing, but it gives extra focus to langs my.
The original sentence, Die passasier sit langs my, is the most neutral and straightforward version.