(quoted from post at 13:54:03 06/15/17) Below is a link to a wiki article about Union Pacific 844.
844 is also a 4-8-4 Northern locomotive but built later in 1944. She represents the absolute zenith of steam locomotive design in the US, as such design efforts ended in the mid 1930s as it became apparent that the future was in diesel-electric locomotives.
Note that 844 has roller bearings throughout, a significant improvement from the bronze bushings used in earlier locomotives, as roller bearings required less maintenance.
844 is unique as she is the only steam locomotive never taken out of service by a major US railroad. Currently, undergoing overhaul, she will soon (if not already) be back in service on the UP. She operates mostly excursion service but occasionally pulls freight trains.
High drivered and somewhere around 4,500-5,000 HP, she is capable of speeds well above 100 MPH. No one really knows how fast she can go because conditions do not allow such speeds these days.
There is plenty of fascinating utube footage of 844 available. Google is your friend.
Dean
Union Pacific 844