Railroading basics - you need some kind of "authority" to occupy any railroad track, or take the track out of service.
That "authority" can be somewhat extensive such as a dispatcher, signal system and PTC (positive train control); to the extremely simple. The simplest form is "other than main track" which means you have to operate at a speed prepared to stop within one half the range of vision short of train, engine, railroad car, men or equipment fouling the track, stop signal or derail or switch lined improperly. "Restricted speed" includes the same wording but adds some more provisions. These terms are defined by federal regulation.
On uncontrolled track where a dispatcher has no direct authority controlling movements, such as on a main track in Yard Limits, within Restricted Limits, on other than main track including yard and industry tracks or Industrial Leads, everything is operated with the stop within half the range provision, much like driving through a busy parking lot with limited visibility and very poor brakes.
Even if some kind of a more formal authority is required, there is still typically a provision allowing joint occupancy at restricted speed. Joint occupancy wouldn't provide acceptable safe working limits for the track crew, so additional protection such as the portable derail would still be required.
In this instance, I would guess the investigation will include:
What authority did the train/engine and track crew have to occupy the track?
If the train/engine was not required to stop within half the range of vision, did the track crew have a track bulletin or other formal advance warning of the potential existence of a derail or stop signal?
The one thing I never cared for is the use of the small red derail sign versus a larger, more visible typical red board.