OpenTTD 3D status
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The Design Document is a paper intended to describe the whole game in detail. Currently, it's quite outdated and does not really reflect the development. However, efforts will be made to make it useful again.
The project's SVN repository is at svn.transportempire.com
There is also a web interface.
For a list of TE developers, check the human resources wikipage.
In the past, TE was widely discussed on the forums and IRC. In this directory you can read logs and minutes (summaries) from the IRC meetings. However, consider them an archived material due to their age and the fact that the current TE development does not reflect them much.