As it would seem in the picture Larry posted here [
ngdiscussion.net] She didn't fall streight into the pit! If you look at the pic you can see the turntable railing is broken up to the pilot. This means that she walked along the edge of the table before she just fell over.
This also means that she didn't need much speed to get that far into the pit. Larry also posted a news paper quote that said the guy that was on the loco when she started to walk jumped off. Even without air he could have put it into reverse.
If you read els where in that thread it is stated that she had a leaky throttle, no air, drivers not chained, and the cylinder cockes closed.