I just had my wife's 400 apart for a chain replacement, and its rails were also exploded, INCLUDING the bottom end rails.

The lower ones are not supposed to go bad, according to my tech. Their failure may be related, if your uppers were replaced 60K ago. Failure of the uppers may accelerate failure of the lowers.

I'd at least inspect them, and if you have the time and energy, pull your front cover and replace the rails at the bottom. It's a bit of work, and some might argue against it on that basis. Your call. I'd be curious what your lower rails look like.

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Peter
1985 300TD 4-speed 212K
1992 400E 343K
2001 E320 72K