I mean, Doc brown did tell Marty the neccessary parts to repair wouldn't be invented until the 1950's at the start of the film, but maybe he just gave up too quickly because he enjoyed living in the past? Mrs. Clara and her desire for adventure could well have encouraged him to give it another go.
There is one little problem though. The tme machine could only be repaired in the 1950's because the time curcuits were powered by Mr. Fusion which was still working. How did the doc generate the 1.21 gigawatts neccessary to power the time circuit in the train? Not with steam power!