Any one have ideas of the most likely routes?
Think of it like current day airlines. Each race will have a flag carrier (equivalent to BA, KLM, etc) Which will run routes from one homeworld to others nearby. There might be codesharing, like it's the Czech airline that runs from Praha Airport to Schiphol and back but under a KLM flight code.
You'll have your major routes inside races borders, like your Glasgow to London, or NY to Washington) and you'll have your local flights say from Earth to Io, like Glasgow to Edinburgh.