how to implement gpx-files in OSM

If I implement a gpx track from a file of my harddisk to an OpenLayer it works great as long as the gpx file is in the same directory or in a directory down. I receive the map from the internet and the track from the harddisk can be seen.

var GPXVariable_1 = new OpenLayers.Layer.Vector("Alpspitze", {

strategies: [new OpenLayers.Strategy.Fixed()], protocol: new OpenLayers.Protocol.HTTP({
url:"Alpspitze.gpx", format: new OpenLayers.Format.GPX()
        }),  // works

... url:"directory_x/Alpspitze.gpx" // works

If I try to implement a gpx file that is in a different directory structure, it doesn't work.

... url:"../../directory_y/directory_z/Alpspitze.gpx" // doesn't work

What am I doing wrong? What is the correct syntax for the address?