It depends. If the primary principle is that retirement is necessary because of the wear-and-tear on one's body, then, yes, we should have some way to gauge that wear-and-tear. It seems odd to me that we can use this as the primary mechanism to justify retirement but also push back on the need to measure it.

As far as this leading to diminishing the collective bargaining of the unions, the unions are in the drivers seat there. I prefer to think of union members as smart enough agents to understand and weigh those ramifications. If it's in their best interest to maintain solidarity, I'd hope they would. But maybe you're right and it plays on short-sightedness inherent in people.

FWIW, I am in favor of giving sick days to the rail employees. It seems a bit absurd not to. I don't know the particulars of the way PSR works, but it seems like the objective function/constraints need tweaking to capture the negative externalities to employees. To me, it's the govt's role to ensure those negative externalities are accounted for and they probably have a number of tools to accomplish this, up to and including regulation.