Seriously, watch them just as they come out from behind that big silver box on the left. They take a big step down.
I had not looked that closely, but doing so just now it is quite obvious that is where the issue starts.
Without further information I was put the cause on 'Bad Tie Condition'. Such that the rail spread creating 'Wide Gauge' or a rail turned over account the ties didn't didn't hold the spikes well enough to prevent it.
