Hi Jason,
You need the content creation guide. 2006 is basically the same as 2004 (and probably 2009 too)
This is the only official information:
[img:jldnb5zk]http://www.auran.com/TRS2006/global/files/CCG2006.zip[/img:jldnb5zk]
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Otherwise the forums on [img:jldnb5zk]http://www.trainzdev.com[/img:jldnb5zk]
<!– m –><a class="postlink" href="http://www.trainzdev.com">http://www.trainzdev.com</a><!– m –>
may be another source of info.
I think the only mandatory ones are the a.limfront and a.limback