With such an inherently anachronistic concept as "trains in space," it can be difficult to pull it off in a way that isn't comical to the point of disintegrating drama. This work not only has excellent production values, but the characters are all well-developed, the technology does not break suspension of disbelief, and the deaths are all prepared well enough to evoke a real emotional reaction. To my surprise I actually enjoyed the episode "Tranquility" where nothing of importance occurs - it serves as much as a much-needed break for the viewer as for the heroes, and dives into their social relationships in more detail.
I can raise some minor criticism. A number of loose ends are left: the Supreme Commander and her ability to see the future is never adequately explained, nor why she fails to use it to the SDF's advantage, and a certain primary character's killer is never revealed. It's also difficult to interpret the Supreme Commander's tears as sincere. Some of the concepts, like the idea of convincing humanoid robots with emotions, really could have used more gradual introduction, as this is advanced technology even for the setting.
But enough nitpicking! If trains in space sounds like fun to you, watch it, all the way through.