I am not from/in Phoenix so I don't know... but isn't this what you're looking for? http://www.valleymetro.org/nextride

  The NextRide electronic service provides quick access to
  next bus and train schedule information.  All you need is
  a phone or access to the Internet, your route number and
  your STOP# (located at every bus stop and light rail
  station).
  
  Valley Metro bus and train vehicles are equipped with GPS
  technology allowing you to access real-time scheduled
  arrival times.
  
  ...
 
  1-15 minutes before the scheduled arrival time, real-time
  departure times are supplied... 16-90 minutes in advance,
  scheduled times are supplied.

Thank you!

Before making the comment I searched around but failed to turn up that info. I knew that the bus stops all have those STOP#'s but had never spotted one for the light rail.

Tested this out on my 2 rides on the light rail yesterday and it became clear that the times on the web site aren't updated live. They appear to only ever return the scheduled time, not how the train is actually running :(