The final version of the chain brake was automatic, held off by a tensioned chain (spring, I think). Apparently was got to work on the North London Railway not sure how successfully.
As far as the accident that started this discussion, you could argue that the automatic vacuum brake failed utterly, as it was inoperative! That the strings had to be pulled and disabled it during shunting was a defect that the chain brake did not have. Not that I am advocating the use of the chain brake, just pointing out that things are not as black and white as they are sometimes painted.