They now Uber the check from one bank to the other, instead of the previous mechanism of sending it via pony express.

Thanks to truncation, the pony express was replaced with faxes a few years ago.

That's part of it, but the mandate effective in March 2018 is also part of the "same day" speed.

https://www.nacha.org/rules/same-day-ach-moving-payments-fas...

Thanks, I was just reading the parent link to part 2. Makes sense they are making lower-risk / more trusted transactions faster, similar to remittance companies already.