It's worth clearing up some misconceptions here.
1. There are plenty of passenger operations with transit coupler running on trackage with equipment running Janney couplers. South Shore, Metro North, Jersey Transit are the biggest examples.
2. There are coupler adapters, every Jersey Arrow and M8 carries one on the A-end. South Shore diesels carry them (as there are 15 diesels and 70+ coaches) See video: https://youtu.be/7hoMkTpSQHw
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3. Someone implied there is a regulation about not mixing transit couplers on mains with freight. The above would imply this is wrong. Some members have told me that the reason NJT carries adapters on every MU is because of an FRA reg, but then why doesn't South Shore or Metra Electric carry such adapters on their MU's?
4. Although it isn't in the USA, the San Martin commuter trains in Buenos Aires are seven car sets with one double-end locomotive. All have transit couplers. At terminals, the power cuts off and runs around and the whole process is very fast. I'm not advocating this to happen in the USA, but worth noting that it does happen, and it works well.
Would it make sense for Amtrak to adopt transit couplers? The big answer is no, because it makes sense in the corridors but not on the long-distance trains. Since they share power, terminals, and rolling stock, it would be hard to make that work.