Noted this a few weeks back, 
My best guess is that this section of the line is shared with the line to the Prax Jarrow Terminal. 

The metro line used to be separate, but was changed a few years back, since it used to be a single metro track. 

 (you can see the branch near the Tyne Tunnel / A19). 

So the track is both freight and Light rail. 

Hope this answers your question. 