I have made double tracks. Basically you need to make the terminal stations at each end single track, then just drag one track into the other with a switch just before the terminal station. Don’t forget signals of course, place one way signals at regular intervals along each track, the number needed depends on the number of trains you intend to run, for more trains use more signals. If you place too many signals then trains are more likely to bunch up. By using good signal spacing you can keep the trains apart. Don’t place any signals between the end of the double track section and the terminal stations. I am British so I run my trains on the left but you can run them on the right if you wish. If running your trains on the left then the signals should be in the “six foot” (the space between the two tracks), if running trains on the right then the signals should be on the outsides of the double track.