hi. you are correct. 'opened' is not acceptable. '... waiting for it to open' is fine. you can't be 'waiting for it opened'. opened is the past tense of open, meaning the door already is open... then what are you waiting on?
'waiting for it to be opened' is not grammatically incorrect, but it is not the best way for that sentence. that sounds like it is from a command or instruction, "the train door is not to be opened under any circumstance!"