When Marty travels in the DeLorean to 1885, he finds himself chased by Indians. An arrow is fired into the fuel tank, which duly runs dry. As Doc states "of course without fuel the car will never hit 88 mph !" (88 mph = 141.6 km/h)
It's hard to see how the DeLorean could be made to attain such a speed in 1885. You could...
_ Pull the car by horse.
Impossible. Even the quickest horse on earth can't run faster than 24
mph.
_ Roll it down a steep hill : again impossible, since no surface is
smooth enough.
_ Replace the fuel with inflammable spirits. However, as Doc and
Marty will realize, the stuff is a little bit strong : The fuel
injection explodes !
At the time, the train is the fastest vehicle on earth. The plane has yet to be invented and other means of transport are way behind where top speed is concerned.
It's not that straightforward, though, since no one has yet managed to drive a train at such a speed in 1885.
As the train drivers says, a locomotives cruising speed when pulling several wagons is 50 mph (80.5 km/h). According to him, one Fearless Frank Fargo has reached 70 mph (112.6km/h). It actually wasn't until 1890, in France, that a speed of 90 mph was reached when a Crampton locomotive beat the speed record of 144 km/h between Montereau and Sens.
Doc and Martys aim of reaching 88 mph is therefore theoretically possible, as the train driver predicts.
When Marty go back to 1985 using the railroad, the Delorean is destroyed by a train coming from the inverted direction. Then, it seems there's no way to come and get back Doc, who remained with Clara in the Old West. But Doc is a very talented inventor : he comes back to the future the same day on board of a new time travel vehicle : a locomotive !
Then, we learn he has already gone to the future to hover-convert the machine. Now, the train can fly !
This last part of the
trilogy makes one wonder where did Doc find enough time and money to
get a locomotive and to change it into a time machine, with the ways
and means of the Old West.
Anyway, you can notice on this picture that the flux capacitor is
still essential : it's just in front of the chimneys. Another funny
thing : the doors of the cabin open the same way as the DeLorean
gullwing doors ! That's why some Back to the future fans call it the
"DeTrain".