OK, right. I think those SP locks you refer to are occasinally refered to as 'banjo', probably due to their appearance. I know of just a few places that they can be found, none of which are important to operating the main line.
Those big locks that materialized in the last 7-10 years are troublesome to operate, and like you've pointed out, fail after high use. Is anything made to last anymore? The brass key you mentioned is the same AT&SF cut, albeit stamped BNSF.
What I find funny is that often the big, troublesome locks with the massive hardened shackle are applied to some antiquated, weak and rusted out hasp. Think: the weakest link!