。
She left her umbrella on the train.
Literal
She [topic-は] train [possessive-の] inside [in-に] umbrella [object-を] left-behind.
置き忘れる is a compound verb: 置く ('to put/place') + 忘れる ('to forget') = 'to leave behind by forgetting.' It's the standard word for accidentally forgetting something somewhere. Written in hiragana as かさ rather than the kanji 傘 — both are common. Forgetting umbrellas on trains is a stereotypically common occurrence in Japan.