Mr. Cole,
May I ask what UP passenger equipment you desire to model?
1) Original UP passenger trains from the mid 1940s through the mid 1950s, which would/could have been powered by steam?
or,
2) The excursion era powered by UP 8444/844 or Challenger 3985?
Should you desire the "original era", then I would wait for Sunset/3rd Rail/Golden Gate Depot to develop the appearance of PROPERLY riveted car construction for prototypical UP SCALE passenger equipment.
One thing I forgot to add, the MTH model of the UP 844 is truly accurate for the "modern excursion era" (note the twin dynamos on the Fireman's side forward of the cab), while the Lionel Legacy version (previously issued) is an EXCELLENT representation of the 844 when used in regular service after she was up-graded to the Worthington Type S Feedwater System in about 1952/1953.