[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?

[wideminded]

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

The fact that they were empty should make the brakes work better. Train cars are supposed to have brakes that turn on when they lose air so that they won't roll downhill if they are accidentally detached. Maybe someone who knows more about trains than me can comment on how this happened.