Future Trunks became a Super Saiyan in his own timeline. In that timeline Trunks was trained by Gohan a lot. Trunks almost accomplished Super Saiyan but failed. Until Gohan was murdered by the androids and saw Gohan lying faced down and dead in the Rain. Because Gohan was his master and friend it drove him to a new height and with the pain of loss he became a Super Saiyan!
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