In "Back to the Future 3", why did Doc. Brown not build an electric motor instead of trying this steam powered locomotive gamble?

Nostalgia.

Because if that happened, we would have never heard Marty say, ’‘he’s an asshole!!’’

Also I think they were shit out of luck as far as time went. I have absolutely no idea what the hell you’re talking about, but I’ll just go ahead and say, it would have taken too long.

Because the script tends to dictate the plot/dialogue, even in those piss poor films.

Electric motors are not that easy to build. He would need various metals and equipment to produce all the parts required to transfer motion to the wheels.

@mattbrowne
Remember that massive behemoth of a machine he built just to make ice cubes?
I doubt he would have had an issue with doing the same to build an electric motor.
And besides, he built an entire time travelling train later on.

@ragingloli – You’re right. There are too many inconsistencies.