Have a look at http://www.cferrero.net/maps/guide_to_mkgmap_style_files.html
For the list of standard line types, read http://www.mail-archive.com/mkgmap-dev@lists.mkgmap.org.uk/msg01125.html

On the railroad resolution thingy: your Oregon behaves just like my GPSMap76csx: railroads (0x14) appear at 200m zoom. Garmin GPS units have internal rules about what zooms different things should appear, and these rules can over-ride what you set in the mkgmap style file. It appears that 0x14 cannot appear before 200m, no matter what you do. The only way to overcome this is to use a different type code, as you did with 0x39. I would suggest, however, that you use an extended type code (e.g. 0x10100) for railroads rather than 0x39, for the reasons given above.