In the options-display menu there is a setting for translucent Copper. For some reasons enabling this option and/or changing the transparency has no effect. The selected layer stays visible and the layer under it is invisible.
Chris, if you wish to make the Tracks semi-transparent, try using the Merge Colours (Options, Display) . You will need to switch off the Enhanced Graphics option (Options, Display) if you wish to use the Merge Colours option at present.