Hi Transportfan
The different names were 'localizations' of the same software (in some cases, these had the language changed, in others they were purely name changes by the regional publishers).
There have been some 'regional' editions, which included different content. The TRS2004 'deluxe' editions were examples of this. Generally these 'regional' editions are only released if a publisher is wanting to release a region specific edition, such as 'TRS2007' or 'TRS2008' (TRS2008 being TC1/2's game platform, with a European specific content set).
In the case of 'engineers edition'. This is the 'TS2010' release of Trainz (known in full as 'Trainz Simulator 2010: Engineers Edition'). This was originally going to be known as 'TS2009:Engineers Edition', however as it was released later than originally planned, it had the name changed to TS2010. It was originally going to be the second half of the TS2009 release, having the new content included, but no surveyor (making it a 'driving' edition). This then changed to a 'full' edition of Trainz (new content, and included surveyor, as well as a new features, etc), and was renamed to TS2010:EE
For most releases of TS2010, the name will be 'TS2010:Engineers Edition', since this is the name for the international release. Some localized editions (language changes only) may see the 'engineers edition' dropped from the name.
If the US publisher were to organise for a 'regional' release to be released, this would likely end up with a modified or different name. However, this would only occur if the US publisher were to organise it.
As it is, much of the new content in TS2010 is focused on the US (the 'ECML' route is the only non US/Candadian style route, with all others being either US style, or US based maps).