What was the Union Pacific Railroad Strike of 1855?
I wonder if you might have made some mistake in this question. The Union Pacific Railroad did not exist (at least not by that name) in 1855 and 1855 is not a year that is particularly known for its railroad strikes.
The Union Pacific Railroad was not actually created until 1862. It did absorb some other railroads that did exist before then, but there was no company called the Union Pacific until 1862.
Furthermore, there were not any major strikes (at least not major enough to get into the texts I use to teach US History at either the high school or college level) until the 1870s. So I wonder if you might have made a mistake in this question.