When -[y] ken is used following the future ending, the combined form means " while/just as intending to do something " and the second part of the sentence generally shows that the intention has not fulfilled ;

Ankara´ya trenle gidecekken son dakikada vazgeçtik, otobüsle gittik.

While we were due to go to Ankara by train, we changed our mind at the last minute and went by coach.

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Ankara´ya trenle gidecekken otobüsle gittik.

While we were intending [due] to go to Ankara by train, we went by coach.

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Kapıyı açacakken pencereyi açtı.

While intending to open the door, he opened the window.