。
She may have missed the train.
Literal
She [topic-は] train [to-に] ride-was-late [might-かもしれない].
乗り遅れる is a compound verb: 乗る ('to ride/board') + 遅れる ('to be late') = 'to miss (a vehicle) by being too late.' ~かもしれない wraps it in speculation. A compact and natural sentence for a common situation.