The TrainzItalia Challenger is the one asked about by the OP
Shown here with one of anl's FEF's
All the TTG-Y now available in trainz
More information on the UP Two Tone Gray Paint
http://utahrails.net/up/ttg.php But long story short; For passenger service certain engines in four classes were painted TTG with yellow stripping in 1946 then repainted to TTG with "silver gray" stripping in 1949 and repainted black in 1952. Specific engine numbers are given in the article.
As to the size of the engines;
These engines are slightly taller than anl's early Challengers and the Big Boy
From the front you can see that the diameter of the barrel is about the same and it appears as though the barrel is somewhat higher above the drivers. (The Big Boy is the one in the center)
From above you can see that the late Challengers are about a half cab length longer than the early Challengers although still shorter than the Big Boy.
Rudolf3610's engine #3979 comes with a coal tender however this engine burned coal for less than 2 years mid 1943-1945 before being converted to oil. #3985 comes with an oil tender but this engine never burned oil as only 3975-3984 were converted to oil. I'd suggest swapping tenders and repainting them. (source
http://utahrails.net/steam/up05-1915-1962-15.php)
BTW one of anl's Challengers is numbered 3939. The early Challengers were renumbered in 1944 from 3900-3939 to 3800-3839 so if you are using the late model Challengers you shouldn't use this engine or you should renumber it to 3839. (or renumber the rest of the early Challengers if you are pre-1944, although then you'd have to convert them to coal.)
Probably more than you ever wanted to know about UP Challengers.