It's an exercise from my textbook where I have to fill in a word from a list. The list is: なる, 渡る, 話す, できる, たいへんな, 降りる.
The sentence in question is as follows:
電車を _______ としたら、ドアが閉まってしまった。
I don't think that と+したら could make any sense here, but my textbook hasn't taught me the word としたら http://jisho.org/search/toshitara yet so I can't rule it out. That's why I'm asking you what you think.
I would insert 降りる here and translate the sentence as follows: "If it happens that I exit the train, unfortunately the door will close."