I'm not sure about the morality of it; I think it would be a matter of whether you are okay with taking on the responsibility of playing God in the fate of unknown people, when the primary decision was already made for you (ie. the brakes failing).

If you are comfortable taking responsibility for actively killing 1 person via your tangible action, then turn the trolley. If you are comfortable with watching 5 people die because you didn't want to interfere with what was inevitably going to happen, then don't!