My recollection is that there was a serious wreck some years ago that the NTSB determined had a "contributing factor" of a blocked brakeline. I believe that this accident was the December 14, 1994 wreck investigation linked here:
[
www.fra.dot.gov]
While the report makes no mention of this, I seem to recall allegations that "someone must have tampered with a brake cut-off valve (between cars)" while the train was stopped at the summit.
Hence the increased security.
I might be mistaken about which specific wreck was the precursor of the "security area", but I believe the general circumstances are correct.