Apart from it being a plot contrivance, a possible answer to the question has been offered by IDW comics, namely Back to the Future: Tales from the Time Train.
Longtime fans of the franchise have posed a crucial question at this juncture: how was Doc Brown able to create a flying steam locomotive equipped with a flux capacitor without the necessary components required for it to function? As electrical components had not been invented until 1947, Doc’s ability to create a train time machine creates a probable plot hole-of-sorts when viewed from a practical standpoint. During this endeavor, Doc encounters Clara Clayton (Mary Steenburgen), who is later seen accompanying him in the train time machine towards the end of the film. After the DeLorean is unable to reach 88mph for it to be able to travel back in time, Doc devises a plan to use a steam locomotive to push the DeLorean up to the requisite speed. Fox) travels to September 2, 1885, to save Doc Emmett Brown (Christopher Lloyd), who is purportedly killed by Biff Tannen’s (Thomas F. In Back to the Future Part III, Marty McFly (Michael J.