If only the morning subway weren't so crowded.

Literal

Every-morning [possessive-の] subway [subject-が] that-much crowded not-were [wish-なあ].

~なかったらなあ expresses a wish contrary to fact — 'if only X weren't the case.' なあ is a sentence-final particle conveying wistful longing. あんなに ('that much') emphasizes the degree of crowding the speaker is lamenting.