1 回答
Only once I had managed to jump on the underground train as it was just about to leave出発しそうだった地下鉄に乗った時点で
did I have time to take in my appearance, reflected in the dark carriage window. 自分の見た目を暗い車両窓で見る時間ができた
小説等の文章？と想像しますが、よりシンプルに言い換えれば、Once I had jumped on the underground train, then I had time to take in my appearanceになります。それでわかりやすくなりますでしょうか。