Need to get trains from a depot onto both lines, now.
You just need to build a crossover (a track leading from one of the two tracks of a double-track line to the other) just after the point where the track from the depot joins the first track of the double-track. That way, it can easily cross onto the second track. As long as you put signals both before and after the two switches on the depot-side (but not on the track leading to the depot), and before and after the one switch on the far-side, then the path from either side to the depot will be two way (ie. the entire signal block will be two-way), whereas the rest of the double-track should be one way each side if the signals are set up correctly.
If you want the trains to be able to reach the depot from either direction, you need to split the line leading from the depot before it joins on to the main track, and the new branch will join the main track facing the other direction (where you repeat the steps mentioned before to mirror what you did previously).