。
I'm supposed to meet her at platform 8.
Literal
I [topic-は] number-8 platform [at-で] her [with-と] meet [thing-こと-に] become-am.
~ことになっている is the standard 'it has been decided/arranged that' construction — circumstantial or arranged outcomes, distinct from ~ことにしている (the speaker's own habit). Often translated as 'be supposed to.' The と after 彼女 marks her as a co-participant in the meeting, not に — using と with 会う carries the nuance of 'meeting up with' rather than just 'meeting [a person].'