The slow speed release of tripcocks you mention is known in the UK as Moorgate Release - after the terrible collision at Plat 9 in Moorgate, when the London Underground operated it.
I used to drive into Plats 9 & 10 when I worked for Great Northen. You had speed boards bringing you down from 30, to 20, to 10 on the approach to both dead_end plats. As you rolled down the platform a timer counted down, and you watched nervously for the trip arm half way down the platform to drop. I never did more than 8mph myself, because you just don't need the crap that goes with a stopped train in the rush hour, OR a safety incident on your licence.
Cheers, John
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