They are all set to rockdown for a reason. In the latest build tram got some odd bug, I assume its a hole in the occlusion stuff, which makes it lag way more than rockdown. As most people want lagfree games, they set their servers to the lagfree map.
Since tram is quite a large map, it is better suited for 8vs8 games, however since we can barely play 5vs5 without major lag, rockdown seems to be the only real choice to allow for some proper gameplay until more performance optimizations come into play.
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In the latest build tram got some odd bug, I assume its a hole in the occlusion stuff, which makes it lag way more than rockdown. As most people want lagfree games, they set their servers to the lagfree map.