These are the “Big Guys”, the primary love of many railfans. These are the restored steam engines that pull full length excursion trains out on the main lines of Class 1 railroads. Differentiated from tourist line steam engines, these engines are generally classy looking, fully operational, well financed and meet all the federal laws to operate commercially on the class one railroads. This is a small list and there are a few engines that qualify for this list but run less frequently than the others, and escape my memory as I compile this list. I will add them to the list as I remember or run across them again. The Federal Railroad Administration maintains strict laws for the operation and safety of steam locomotives in the modern era. Rules promulgated in 1999 take up 90 pages in the Federal Register. See the link: here. The rules include stipulations for 31 day, 92 day, and annual inspections as well as a 1472 day (4 year) boiler inspection which includes a full tear down and rebuild of the boiler. In addition, insurance cost are astronomical. Logistics of fuel and water need careful thought and many unanticipated other things can throw up roadblocks, but railfans and corporations continue to fund these magnificent machines with donations and high excursion ticket prices because they love steam.
The list below is in no particular order and is by no means complete. I can't remember them all. Senility is fast approaching, (help me here, If you know of a qualifying locomotive not listed here please let me know. See my contact page)
UP 844 from Wikipedia: Union Pacific 844 is a 4-8-4 "Northern" type steam locomotive built by the American Locomotive Company in December of 1944 for the Union Pacific Railroad. Constructed as a member of the FEF-3 class of 4-8-4's, the 844 was the last steam locomotive delivered to Union Pacific. Though the FEF-3 class was originally built for high-speed passenger work, 844 and the rest of the FEF-3 class were pressed into a variety of dual-service work. While commercial Union Pacific steam operations ended in the late 1950s, the 844 was retained by the railroad for special activities. Today, it is one of UP's oldest serving locomotives and is the only steam locomotive never retired by a North American Class I railroad. Originally built to burn coal, 844 was converted to burn fuel oil for convenience. Links: https://en.wikipedia.org/wiki/Union_Pacific_844 https://www.up.com/aboutup/special_trains/steam/locomotives/844.shtml
UP 3985 from Wikipedia: Union Pacific 3985 or UP 3985 is a four-cylinder simple articulated 4-6-6-4 Challenger-type steam locomotive owned by Union Pacific Railroad. It was built in 1943 by the American Locomotive Company. The locomotive is one of only two of the original 105 Union Pacific Challengers in existence, the other being UP 3977 on static display in North Platte, Nebraska. Currently it is being stored. Before it was stored, it was the largest and heaviest operational preserved steam locomotive in the world.
UP 4014 from Wikipedia: Union Pacific 4014, or UP 4014, is a four-cylinder articulated 4-8-8-4 Big Boy-type steam locomotive owned by Union Pacific Railroad. 4014 was retired from service on July 21, 1959 and donated to the Railway & Locomotive Historical Society in Pomona, CA on December 1961. The locomotive reached its destination in January 1962 and was displayed in Fairplex through 2013. Thereafter, Union Pacific 4014 was moved to Union Pacific's Steam Shop in Cheyenne, Wyoming, where it is now being extensively restored back to running condition. When 4014 returns to running condition, it will displace UP 3985 as the largest, heaviest and most powerful operational steam locomotive in the world. Links: https://en.wikipedia.org/wiki/Union_Pacific_4014 https://www.up.com/aboutup/special_trains/steam/photos_videos/bigboy/index.shtml