You can't compare mainline curves to stuff out in the yard.There are no curves on the system that are much tighter than the King to Union curve in Toronto, around 400 feet. At the same time, the storage yard has much tighter ones, and we have not heard of any issues there. These are essentially LRVs, not heavy rail. There seem to be some problems with the type of rail and the way it is laid that are a little too complex for me.
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The mainline curves need to be built to a higher standard, with higher superelevation/cant, and maintained to a higher standard as well. They will see far higher usage and speeds.
Curves in the yards are passed at walking speed and by not nearly as much traffic, and so the track doesn't need to be kept to nearly as high a standard.
This is why a lot of systems will state that their "mainline minimum" is one number, and their "yard minimum" (or something similar) is a different, smaller number.
One other thing to consider - if a derailment does occur, where would you rather have it? (Of course, the answer is "nowhere", but you still need to plan for eventualities.) A derailment on the mainline can and will disrupt service for hours or more. A low-speed derailment in the yard may cause some temporary disruption to some of the fleet getting into or out of service, but the equipment to re-rail the train is nearby and so it shouldn't be a long process to get things back to normal.