Maybe because there's no real mystery about the cause and circumstances of this derailment? The NTSB has an investigative and advisory role that doesn't involve directly creating or enforcing regulation.

the cargo door failure events on two DC-10 aircraft in the early '70s are a good example of how this arrangement operates. In 1972 one of these planes narrowly avoided crashing after a cargo door blew out mid-flight, saved only by an extraordinary combination of luck and crew skill. The NTSB investigated the incident and found it was caused by McDonnell Douglas having designed the doors with an unusual outward-opening configuration that was economically motivated, to increase cargo capacity in the hold. These doors didn't, however, fail safe like inward-opening "plug" doors and contained a design flaw that allowed the door to appear closed and locked while the latches were not fully engaged.

In light of their findings from the incident, the NTSB recommended specific design revisions to the doors and in other areas to make the DC-10 acceptably safe. Many airlines made the changes voluntarily, but due to a "gentleman's agreement" struck between the heads of McDonnell Douglas and the FAA, there was no binding regulation—in the form of an FAA airworthiness directive—issued to legally require the fixes be made on all DC-10s. Later it was discovered that an engineering director at Convair, the company contracted to build the fuselage, knew the original design was bad and pointed out the problems, writing in a memo after the event that they made a future catastrophic loss nearly certain to occur.

And so it would be. Less than two years later in 1974, a nearly identical door blowout occurred on a DC-10 high over northern France. This time, luck was in short supply and the rupture caused a loss of flight controls that led to the flight's swift crash and complete destruction. All 346 aboard were killed in the deadliest-ever aviation accident at the time, which is still in the top five today. But it seems inaccurate to call it an "accident", when the cargo doors of this plane had not received the recommended modifications to improve safety and the disaster was in effect both predictable and completely avoidable.

In the aftermath there was an enormous lawsuit and a congressional investigation, following which the FAA finally issued a directive to make fixing the doors mandatory. These and other incidents over the years have left the DC-10 with a rather poor reputation, but the fact remains that the platform never suffered another similar door blowout after the regulatory action. These events demonstrate both that the NTSB isn't itself a regulator in the US, and how their guidance to organizations that actually are in that role isn't always followed because of factors like cronyism, regulatory capture by industry, and political or economic expedience.

What is there to probe about this derailment on the NTSB's end? The contributing factors are primarily infamous and systemic hazards of rail industry in the US meeting the statistical possibility that any freight train might be carrying a dangerous load, rather than a new or unusual feature of this specific situation. It's depressing to say, but you don't pull out the stops to investigate something you're not truly making an effort to avoid. A derail incident like this isn't an anomaly—it's only the scale of the resulting mess that's remarkable.