Trainz Railroad Simulator 2004 incorporated a load of technical changes and these required many bug fixes that were released as the four service packs released through 20. Trainz Railroad Simulator 2004 was the first version of Trainz to include interactive industries and loadable rolling stock capabilities, which allow the player to deliver various resources (such as coal, wood, and steel) and passengers to various industries and stations across the playable area. There were many industries included and much of the bundled rolling stock was updated to include this new dynamic loading and unloading animated capability with a corresponding load-state dynamic driving physics change in the handling of a consist. It was also the first version of the Trainz series to have built-in steam engines. Trainz Railroad Simulator 2004 Deluxe was a later 4 CDROM follow on with the first two service packs pre-installed. It also included PaintShed, and a bonus content CD. This edition was also the first to support downloadable fan-made content, such as characters, buildings and scenery from the popular Thomas & Friends franchise, which has since seen the birth of a massive community on YouTube involving users recreating episodes of the series using Trainz, or creating their own original stories.

Trainz Railway Simulator 2006 (known as Trainz Railroad Simulator 2006 in the USA), or TRS2006, was a transitional release, incorporating the stable Auran JET based TRS2004 GUI modules with only some graphics improvements, but introducing the data base manager Content Manager Plus (CMP) as a new core technology.

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