。
She was in such a hurry that she ended up leaving her umbrella on the train.
Literal
She [topic-は] very flustered-was [because-ので] train [on-に] umbrella [object-を] left-by-mistake-ended-up.
置き忘れる is a compound verb fusing 置く (put down) with 忘れる (forget) — capturing the specific kind of forgetting where you set something down somewhere and walk away without it. ~てしまう adds the regret/completion flavor: 'unintentionally ended up.' The combination 置き忘れてしまう is the perfect verb for the very Japanese phenomenon of leaving an umbrella on a train — common enough that lost-and-found offices at major stations stockpile thousands of them each rainy season.