I presume you have one of these three routes? Essential Requirements DTG Chatham Main Line AND/OR DTG Chatham Main & Medway Valley Lines AND/OR DTG Chatham Main Line: London Victoria & Blackfriars - Dover & Ramsgate
I read it - did you before purchasing the AP 377, 387, 379 pack? You don't need all of the routes but unless you have at least One then the AP pack Won't Work. So I asked you if you actually owned any of those routes Or is that a difficult question?
When you apply the AP Sound pack there is a section that copies files to the new paint jobs If you don't allow that then the installation is not complete
I would redownload the Chatham Line then download the AP pack again to make sure you have a good install. Unlike having just Steam product, as you start to build up 3rd party material sometimes the interaction of the software can go slightly wrong and create conflicts. Best to create a clean point and reload those packs again in the correct order as some files WILL be overwritten and take code from some and splice into others. Respectfully