Apparently it is way, Way down. Per the USGS the lake bottom and most nearby terrain is a very thick ‘mud flat’. The tremendous weight of the causeway rocks and ballast causes continual subsidence.
The predessor trestle was also a maintenance headache. The company that recycled the pilings has a detailed history published on their website.
BTW a Google search on That Other Site will get you over ten years of GSL anecdotes by present and former RR employees.
UPDATE: found some technical discussion on a 2012 trains.com thread (link). Memory must be going … [banghead]