Wheelnrail:
The asset you are missing is an AC44 UP interior kuid 52779:55015 created by Mark Hoffman who was at Digital Roundhouse which no longer exists. I was disapointed when I discovered Frank McCall who creates such great rolling stock installed a dependency that is nearly impossible to find. You may be able to get the interior by going to ProTrainz and downloading the UP AC4400ac. I'm not sure but it's worth a try.
BTW, you need to install Trainz Objects to help you find missing asssets.