The substance of Franklin’s reforms was fairly simple. The post office he inherited was a barely functioning mess, and most post was carried privately by merchants and travelers. Franklin’s basic innovations were straightforward: He had postal riders travel by night as well as day, as you noted. He also penalized postal riders for delays, and he had post offices tally and list letters, so that there would be a record of when a letter had been sent, and post riders could be held to account for their efficiency. He also improved the roads, laying milestones along the post roads, so that post riders wouldn’t get lost, and would be able to measure how far they’d travelled. Making the post officer more reliable also made it more popular, and the volume of mail increased dramatically under his leadership.

As for the “remarkable” 24 hour deliver between New York and Philadelphia, I couldn’t find a scholarly source for this fact (I didn't look very hard, to be fair), but I don’t disbelieve it. Moving mail quickly between New York, Philadelphia, and Boston was a huge priority for Franklin, because he wanted the official packet ships that carried mail between New York and London to be able to serve all three cities. Franklin had the packet ships wait for several days at New York, which meant, if he could move the mail up and down the eastern seaboard quickly enough, a letter from London could get from New York to Philadelphia, receive a reply, and then the reply could get back to New York quickly enough to go out on the same ship the original letter came in on.

But really, 24 hours between New York and Philadelphia is not so remarkable, if you think about how the post office worked in the colonial period. There was no home delivery of letters. The post office merely transported letters between post offices. If you were expecting a letter, you’d go to the post office and see if it had arrived. And in fact, one of Franklin’s reforms was keeping “dead letters” or unclaimed letters at the post office for one month. So if one post rider goes from New York to Philadelphia once a day, and another post rider goes from Philadelphia to New York once a day, all you have to do to get this amazing service is show up at the post office in New York before the rider leaves, give him your letter for Philadelphia. Then, your interlocutor in Philadelphia just has to pick up the letter from the post office and write his reply before the next day’s post rider leaves.

The post office no longer provides a service like this, because it is a much, much, much more efficient system designed to move millions of pieces of mail, not only between post offices, but between every address in America. Franklin’s colonial post office faced a much simpler task. That said, if you did need to send a letter between New York and Philadelphia and get a same day reply, you would have no problem hiring a courier service to do it for you. Before the internet/fax machine, I would assume that a huge amount of business correspondence went back and forth in just that manner on the trains that run between the two cities.

Sources: Kathleen Grandjean, American Passage: The Communications Frontier in Early New England (2015) (this book only deals with Franklin briefly in its conclusion, but it was handy as provides a very efficient summary of his reforms).

Rohit T. Aggarwala, “‘One Chief Letter Office at New-York’: Packet Ships, Imperial Administration, and the Geography of the American Post, 1692–1783” (2019)