I loaded the Pacific Surfliner addon from the marketplace along with the San Diego and Los Angeles commuters and the BNSF ES44DC loco. But the BNSF won't load for almost any of the scenarios. I get this error message saying 'Can't load rail vehicle - MP ES44DC BNSF' and 'Can't load rail vehicle - Auto Rack Car.' Am I just missing an "Auto Rack Car?" I don't see it anywhere in the store.
What have you bought from the Marketplace? The Pacific Surfliner Route is not on the Marketplace https://store.steampowered.com/app/222606/Train_Simulator_Pacific_Surfliner_LA__San_Diego_Route/ Do you have this and have you played the route?
"MP ES44DC BNSF" is a Marias Pass loco. If you don't have Marias Pass, you don't have the loco hence it is "missing" because it is not there.
I feel like the OP bought Larias' game's Pacific Surfliner scenario pack, but doesn't have all the required content for it. Mainly Marias Pass. https://store.steampowered.com/app/1515062/TS_Marketplace_Pacific_Surfliner_Scenario_Pack/ Its not listed very well in the description, but Marias Pass is indeed needed for the scenario pack to work.
I was wondering if OP bought the Pack thinking it came with the route - as he has not bothered to answer we don't know.