。
She couldn't stand the heat inside the train.
Literal
She [topic-は] train [possessive-の] inside [possessive-の] heat [to-に] could-not-endure.
あつさ (暑さ) is the nominalized form of 暑い ('hot'), using the ~さ suffix that turns i-adjectives into abstract nouns. Written in hiragana here. 我慢できなかった is the negative past potential of 我慢する ('to endure/tolerate') — 'could not endure.'