Mercury
Mercury is the closest world to the Sun, baked on one side and frozen on the other. A single day there lasts months, and the dayside is hot enough to melt tin. The Mercurians have made it work all the same.
A vast rail system circles the whole planet. Gigantic heavy-duty trains run along it, stopping at pit stops across the surface. Crews live and work aboard in shifts, while swarms of drones fly out from the trains to mine the surface and return with ore. Along the line, people live in craters dug deep for shelter, and the craters double as spaceports, launching Mercury's resources out into space.
Life on Mercury runs to the timetable. Mercurians are famously punctual and obsessed with schedules, because on their world a late train or a missed window can be deadly. Mining and solar power are the backbone of their economy, and few places in the system have so much sunlight to harvest.
Mercury is a minor faction. It keeps out of the wider politics of the solar system, but it is proud, practical and very much its own people. The Sunward Waystation orbits close by.
Mercury and Venus
Mercurians share a warm, teasing friendship with the cloud-dwellers of Venus. They call Venusians airheads and effeminate, but beneath the banter is real respect and affection. The two don't compete, and they hold a simple alliance against the bigger nations, helping each other out when it counts.