Question: can GPX files contain more than 1 track? I am new to using this format. Since you can name a track, it suggested to me that I could put multiple tracks in a file. However, when I tried testing the file out (for exampe, on http://www.mygpsfiles.com/app/), it only displays one of the tracks.
Yes, the .gpx file format will hold individual features that your gps unit will display as separate tracks. I don't think you can save separate tracks into a single file on a handheld gps unit - you have to do it on a desktop program. I've never tried the website you've linked to; if you can post your data, or describe how you created your .gpx file, someone might be able to troubleshoot your specific problem.