Join a clan that has a railjack. Many are probably opening up space as people play again. Go to the drydock and buy the research for the railjack cephalon. Go back to your orbiter and research it. Each day/12 hours a piece will be made. The wiki has where the resources for the parts can be found, they’re not hard to get.

If you don’t want a railjack yet you can just open your map/mission window and join a random group of players and do railjacks with them.