I get an email from the New York Times each day with some top news stories, and also an “On this day” section. For today (Saturday, May 10, 2014), is has the following:
On May 10, 1869, a golden spike was driven at Promontory, Utah, marking the completion of the first transcontinental railroad in the United States.
I’m not much of a historian, but I remember something in the Mountain of the Lord video that mentioned how the completion of the railroad really helped with the building of the Salt Lake Temple.
The Central Pacific Railroad Photographic History Museum has a short article about the Mormons who helped finish the railroad.