Well, no. There was a version of GMAX which was distributed by Auran (if I have my facts right, and there have been times in the past when I have not) which would produce content for Trainz, but there was an earlier type of file, now obsolete, with a ~.PM file extension, and it may be that this was the type of content produced by that version of GMAX. To the best of my knowledge, no 3D software package, including platform used in house (2012 version of 3DS Max) by N3V, directly produces ~.im files; these are all produced by the importer, although there are dedicated exporters for the platform N3V uses in house. SInce I have only knowledge about 3DS MAX and the dedicated exporters, I cannot say whether the dedicated exporter reads directly a 3DS max file and generates ~.im files, or not.
I also have little knowledge of GMAX, so I'll leave questions about exporting from that platform to others both past and present, to others more knowledgeable. My understanding, however, is that exporting from most, if not all, other platforms is done by means of a custom exporter, which takes the output file format native to that platform, and creates a file in a specified ~.XML format; the importer provided by N3V takes that ~.XML file, and attempts to create an ~.im file from the XML file. Note that in creating content, certain conventions have to be observed in the creation platform. While some of the packages support "Procedural textures", all texturing in Trainz is by means of phototextures, and the names of the textures have to conform to certain specifications.
So the proper way to phrase your statement, is, "if I have a content creator program, like GMAX [or Blender, or Sketchup, or ...] and the proper exporter, I can create an ~.XML file from which the Trainz importer will make the proper ~.im file."
Many fine and noble creators have used, and continue to use GMAX to produce outstanding and valuable content for the benefit of the Trainz community. >OPINION ALERT< However, for an aspiring creator, especially one who does not already have familiarity with a content creation package, GMAX may not be the best platform to learn to use. The biggest problem with GMAX is that all development of the package stopped about 10 years ago, or so. And while the benefit of this is that the platform is now stable, the disadvantage is that, with development having stopped, there is no one who can update the package to handle new features that may be developed as part of Trainz. If you know GMAX already, and are merely new to creating content, more power to you. If you have not used a 3D modeling program before, Blender (an open source, free download from <
http://www.blender.org> costs the same amount of money as GMAX, and is still in active development. While not officially supported by N3V, the cost benefit ratio of Blender is more favorable to the purchase than the platform used in house, which costs more than I've paid for some of the automobiles I've owned. Blender does have a somewhat steep learning curve, but not necessarily any steeper than other packages of similar scope and complexity. And there are ample on-line resources, also free, including a high-school level course, a college level course, a Wikibook (in the process of being updated to the current version of Blender from an older one), and many on-line tutorials available at various sites. >/OPINION ALERT<
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