Are you suggesting the NTSB, Amtrak and other investigative agencies are so inept as to seek input from a railfan forum? Personally I have far more faith in them than that. I feel confident nothing we say or do on this forum will have any impact on their investigation, and to think otherwise vastly over-states our importance.
As a moderator, I'm sure you know our members here run the entire spectrum from those with a casual hobby interest, to those both active and retired from railroad careers. Those careers collectively include experience in operations, engineering, accident investigation, and we probably have some attorneys on here too. The purposes of forums such as this are IMHO two-fold - enjoyment, and education. None of us knows it all, and it is rewarding to exchange ideas and speculation, to learn from other members, and to compare the various information here with that developed by those agencies working the situations in the real world.
I have had experience in derailment and accident investigation. While nothing I was involved with was as tragic as this, I can assure you that our work was done on the basis of cold, actual facts, observed, measured and confirmed in the field and related investigation. To me, the idea of an investigation being influenced by Facebook, a railfan forum or any other unrelated source is preposterous. Pretending we have to "keep quiet" until all the reports are made or all the lawsuits are settled negates much of the value of the forum.
Again, every time someone guesses at ANYTHING, that's speculation - including your guess on the size of the truck involved. Please help us understand what degree of speculation is and is not allowed - the forum rules say nothing to help us decide.