It's not a metric vs. imperial issue really. The numbers they used are what they are. The real question that hasn't been asked or answered in any of the articles I've read on this issue is "How wide are the tunnels"? The safety envelope was based on the old trains. They assumed x for sway, so they needed y for safety, but they don't say how much room they actually have in any of the articles.
Are they just sitting on their butts saying the new trains are too wide based on the old safety envelopes, or do they actually know the tunnels in question are too narrow. The safety envelope and the tunnel width are two separate deals. That would also imply track separation between trains, not just side-to-side/ceiling clearance within the tunnel walls.
Also, does it affect every line, or only certain lines? These things matter.
I have to disagree on the metric versus imperial — Why would anyone intentionally spec a train’s width at 122.05 inches rather than 122 inches? Ten-foot wide objects aren’t measured with micrometers.