[wideminded]

I wonder how the boxcars could run downhill for two miles. It is my understanding that the air brakes on train cars are designed to apply the brakes if air pressure is lost, such as would be the case when a series of cars are decoupled. Here is a quote from a webpage on train braking systems:

Brake Pipe
The pipe running the length of the train, which transmits the variations in pressure required to control the brake on each vehicle.  It is connected between vehicles by flexible hoses, which can be uncoupled to allow vehicles to be separated.   The use of the air system makes the brake "fail safe", i.e. loss of air in the brake pipe will cause the brake to apply.  Brake pipe pressure loss can be through a number of causes as follows:

* A controlled reduction of pressure by the driver

* A rapid reduction by the driver using the emergency position on his brake valve

* A rapid reduction by the conductor (guard) who has an emergency valve at his position

* A rapid reduction by passengers (on some railways) using an emergency system to open a valve

* A rapid reduction through a burst pipe or hose

* A rapid reduction when the hoses part as a result of the train becoming parted or derailed.

[Cincinatus' Wife]

The resulting string of cars — most of them brand-new empty boxcars — rolled downhill for about 2 miles before running into the Union Pacific train as its 30th car rolled by a switch between South St. Mary's and South Presa streets.

Could "brand-new empty boxcars" have anything to do with it?

[endthematrix]

Falling on deaf ears! Great work! I might pose to ask that question to an USDOT official myself.