When the fuel line is ripped why not get fuel from the other DeLorean?
Marty rips the fuel line when he arrives in 1885 and they were out of gas, okay, but why didnt he just ask Doc Brown if they could simply go into the cave where the other DeLorean had been sealed off for the 1955 Doc and Marty to find and use the gas from the tank in there?
Taking the gas out of the tank and transferring it once the DeLoreans fuel line had been fixed would allow them to simply drive back to the future again, no need for the train.
All that 1955 Doc would have to do is put gas inside of the gas tank which is what he does anyway, because Gasoline and other liquids in cars eventually go bad and have to be replaced, you cant leave gas in a car for years and still expect it start up, you have to replace fluids and batteries and other stuff first.