Ok ok, i jsut put my theory to my friend and he has a very valid arguement!
I have just sketched a diagram and found an issue with the plan. Yes theoretically Doc and marty could go back to any period between 1885 and 1955 (using the train) and dig the delorean out of the mine shaft.
The problem arises now. If they travel forward in time from the point of digging it out, they will only be able to travel up to 1955 because... if they have removed the car and gone forward in time, then they will essesentially be travelling forward on the timeline where the delorean no longe r exists in the mine. this would therefore mean that marty is still stuck in 1955. This means that any future would be one where Marty was stuck in 1955 and would have been about 40 years old by 1985 and also trying to avoid the 'real' marty who was born in about 1970.
It would be a similar circumstance to the part in BTTF 2 where Biff steals the Sports Almanac and gives it to himself in 1955 thus creating an alternative reality.
The issue with alternative realities is that travelling forward in time during this period will mean travelling forward in time through the alternative reality as opposed to the actual reality.
Also remember that unlike the Bill and Ted movies where it was believed that time was on a constant loop. BTTF is based on the belief that time is a single line constatntly travelling forward never having been experienced before. If this was not the case then Doc could simply have gone to the mine and dug out the delorean at the start of BTTF 1.