I have the Steam version of TRS19 and as far as I know for TANE and TRS19 all the add-on packages are the same as for the retail version. They all download from the Trainz store. TS12 and before did have add-ons that were installed from the Steam store. I actually preferred that method as it was cleaner and easier to manage them.
Hi RJArtim
The 'steam:' DLC packs in the Content Store are the Steam versions of DLC packs being shown in the Content Store.
This occurs when DLC packs purchased through Steam are transferred to a MyTrainz account to provide access to non Steam editions of Trainz. Unfortunately there is currently no way to make these display the full names in non Steam editions.