。
I saw her on the train.
Literal
Train [of-の] inside [location-で] her [of-の] figure [subject-が] was-visible.
彼女の姿が見えた uses 見える ('to be visible, can be seen') — the spontaneous-perception verb, distinct from 見る ('to look'). 見える takes が on what's seen, not を. 姿 ('figure, appearance, form') is often used as a soft, indirect way to refer to someone being physically present.