。
She kept marching steadily forward through the rain.
Literal
She [topic-は] rain [genitive-の] middle steadily walk-continued.
雨の中 ('in the middle of the rain') uses 中 ('inside, middle') to mark the rain as a surrounding condition through which she walks — not a location in the literal sense, but an enveloping environment. どんどん is a 擬態語 (state mimetic) for steady, rapid, unbroken progression — used for crowds advancing, prices rising, work progressing. Pairing it with 歩き続ける ('keep walking,' compound of 歩く + 続ける 'continue') creates a cinematic image of her pushing through the downpour without slowing. The compound auxiliary ~続ける is one of the productive aspectual elements that combine with verb stems to add meaning — here, sustained continuation.