There's a lot of theories that could fit. Ultimately I think an unaltered 2015 had to exist in order for the hoverboard technology to exist. The only way Doc could have altered the future would be if he made the techology available or known, and more than likely he'd have (somehow) built that train in secrecy to what it actually did, and he and clara visit timelines without disturbing them as much as possible.

Changing the future in the future never made sense anyways. If Doc was willing to go through all that to fix Marty's life, he'd have done him one better and just stopped him from racing and crashing into the rolls royce as a freebie.