Union Pacifc was famous for their huge Locomotives, from Steam to Turbine and Diesel. UP = Union Pacific or Unlimited Power !
Here we have an example of a double big boy and a"Triplex" style power unit.

Andy built this one of a kind "Monster" on a customer's request. He used 3 MTH Big Boys and 2 Rivarossi Tenders. All 3 units are powered and are able to run on 130 cm (51") radius. Total lenght is 150 cm (59"). The time to produce this Giant Boy was unbelievable high.
It was a real challenge to cut the locos and tenders, partially new painting and lettering and lots of adjustments.
Even that kind of loco did never exist in real, we have it now as an HO scale model.