7. Based on my 17 years working as a Trainman and Conductor..
For the MKT, then the UP RR, here's what I think happened:
There are TWO types of brakes on rail cars: Service and Emergency.
Service air is supplied by air from the locomotive air compressor. Each locomotive has an air compressor, but it works only as long as the diesel engine is running. Diesel engine shuts down, no service air. No service air, no service air braking system. Service works by reducing the air pressure via the main train brake handle off the lead engine. When you reduce the main train line pressure, usually by a minimum of 10 pounds of air, the system sets up a corresponding amount of braking force on the entire rail car consist starting with the car closet to the head end of the train. It also set the the locomotive air brakes. HOWEVER, the locomotive air brakes are called independent brakes because they can be set and released completely independently of the train line. When train line air is set, an engineer will often release the independent brake to control the slack in the train couplers. AKA "slack action". When the brakes are released on the service system, the system is re-supplied with air from the locomotives.
When the service air is dumped from the main train line the emergency air brakes on each rail car engage, because each car has its own emergency air reservoir. The emergency air is supplied by air from the locomotive air compressor, and is held on each car on its own reservoir. BUT, that emergency braking air can be totally dumped by manually bleeding EACH CAR via a particular method. It has to be done manually, one car at a time, and it has to be done on the ground. Not from the locomotive. If you bleed a car when it is in emergency there are NO AIR BRAKES on that car. If you bleed the entire train down, there are no air brakes on that train. None.
There is no reserve emergency braking system for locomotives like on train cars. There is air in each locomotive in the main supply reservoirs, but they run off the compressors, and those main reservoirs can bleed down. That is why when you shut down the engine on a locomotive, you set handbrakes on each locomotive, since the air can TOTALLY bleed off the locomotive independent brakes since there are always minor air leaks in the system, and no air is being resupplied by the compressor.
According to the Toronto Star article, there were 5 locomotives, and 72 cars. Four of the locomotives were shut down, and that left one running to keep air on the train.
Here's the main point:
So, that ONE running locomotive was shut down by local fireman when they put out a fire on the train. If the engineer set only the locomotive independent brakes, no service air brakes, and THEN failed to set enough manual hand brakes on the train cars, then there was insufficient braking force to hold the train WHEN the locomotive independent brakes bled down to the point where they released. Because, remember no air was being supplied to them by the compressors. Because the number of handbrakes on the train cars was insufficient to hold the train, it rolled away.