You can also track railroads by using the detailed maps posted here. Go to the page for the particular county and then click "Detail map." The railroad labels shown on these maps are fairly current (well, as of 2005 anyway).
I'm crushed, Luke! Just kidding. I was about a minute late to get a photo of a diesel electric crossing it. But a steam engine is ever better. My photos can be secondary. *smiles*
Libremap.org has topo maps for a lot of the country free to download. They usually have a railroad name from the time the map was made, usually within the last 30 years.
It's Union Pacific. I usually just Google the city name and railroad to try and see if anyone has railfanned there. Or if the state DOT has railroad routemaps.
I sure hope you don't mind that I've set an image of a UP steamer crossing it as the default.
You are also correct on the date cutouts in the portal being construction dates.
Found this bridge looking at satellite images. I managed to get pretty close by driving to the end of Deer River Rd on the south side of the tracks.
This is a massive bridge - but without something to realize the scale it's hard to tell. The end posts are close to 3ft wide.
This is a double track line that is very busy. I don't know what rail line this is, though. How would I figure out that information?