。
She had been coming down the stairs.
Literal
She [topic-は] stairs [path-を] descend-coming was.
Three layers of motion stack here. 降りる is 'descend,' and 階段を降りる uses を to mark the path traversed — the same を you see in 道を歩く ('walk along the road') and 橋を渡る ('cross the bridge'). Adding ~てくる anchors the motion to the speaker's vantage point: she's coming down toward where I am. Then ~ていた makes the whole thing past progressive — 'had been coming down,' a state caught in the middle of unfolding from the speaker's perspective. The result is one of those layered-aspect verbs Japanese builds so easily and English has to unpack into separate words.