The world's largest operating steam train is in Denver. Union Pacific's legendary made a pit stop in the Mile High City on Friday.  

It first rode the rails in the 1940s until its retirement in 1961.

It came to Denver from Cheyenne, Wyoming, and makes the return trip on Saturday.

There is no public viewing in Denver, but the train will stop at in Greeley in the morning.

Big Boy will be outside the museum at the 10th Street crossing for about 20 minutes, sometime between 8:30 a.m. and 9:30 a.m. The museum normally opens at 10 a.m. on Saturday but will be open early for the event.

Anyone going to see the train in Greeley will need to stay back 25 feet or more from the tracks for safety reasons.