Back To The Future Part III The extra DeLorean

So, this is most likely one of the first plot holes many of you would ever have noticed. Marty arrives back in 1885 in a DeLorean. Sadly, the time machine springs a leak and loses all its fuel gasoline proving hard to obtain in the 19th Century. Cue he and Doc creating an ingenious and ridiculous plan to power the DeLorean via a steam train, and various rail hijinks en route. But: why didn t they just dig up the fully fuelled DeLorean the Doc had buried in the mineshaft awaiting his 1955 counterpart? Timey wimey, wibbly wobbly stuff, I guess. More BTTF paradoxes can be found here.