Double Tracks are very useful if you have a busy rail network. Having 2 tracks for your trains gives you higher capacity, the option to build overtaking sections, and some protection against breakdowns. If you want to build a double-track network, you should first decide if the double tracks will be side-by-side, or if there should be a gap between them.
Side-by-side double tracks need less space, but you can't build effective overtaking sections. Double tracks with a gap need more space, but you can build better overtaking sections.
A standard 4 track line
Simple overtaking with 4 tracks
A 2 track line crosses a 4 track line