The British Rail system has a "brand new" steam engine (Peppercorn), so at first glance, it might seem easy for us to do the same thing. It isn't, for a number of reasons.
First, they ran steam on the main lines for about a decade longer than we did, so they had almost another generation of people who knew how to run steam.
Second, watch a you tube video from Geoff Marshall as he reviews their current rail system, and you'll notice modern trains running past Victorian-era stations and signal boxes that are still very active. That's because their historic preservation laws severely limit what they can and cannot eliminate from the infrastructure in the name of modernization. In contrast, over here, when steam left the rails, we were able to build things like left-hand signals, and signals with bi-directional heads, both of which were impossible when steam was the real deal, and which a modern steam engineer would have to deal with. Not to mention modern hig level platforms for accessibility, which a steam engine might not clear. British Rail already had high level platforms back then that were designed with steam trains in mind, while many of our stations had low level platforms then, and were retrofitted with high ones in the last 20 years.
Third, and it's related to the second reason, things like hotbox detectors weren't designed with steam locomotives in mind, and they can and do trip when steam engines travel over them. That creates safety issues.
I don't see how that is a "safety issue", as various parts of large main line steam locomotives have been "tripping" hotbox detectors more more than 40 years, such as SP 4449, UP 8444/844, UP 3985, Mil 261, Frisco 1522, NKP 765, etc., etc., etc.. In most cases, part of the train orders/track warrants, and special instructions in the Employee Timetables, state that the steam locomotive itself is exempt from having to stop & inspect, if the detector finds the problem on the engine.
Fourth, If a steam engine operates over track that has cab signals, the engine has to be retrofitted with a device that can accept them. This might solve the second problem, but not everybody's cab signals convey information in the same way.
Not that big of a deal, as the UP steam locomotives were originally equipped with their cab signal equipment back in the late 1940s & early 1950s, and then up-graded with the current solid-state systems for both UP main lines and former C&NW ATC equipped main lines. Even Ross Rowland's C&O 614 has had the New Jersey Transit high-end cab signal equipment installed, while NKP 765 has had the NS (former PRR) cab signal equipment installed.
Finally, even if we hooked up that new T1 to a bunch of vintage Broadway Limited cars, what would happen when the train reaches Harrisburg? There aren't any operating GG1's left, nor any plans to restore a survivor, and if they did, it would need a whole new set of motors and transformer, owing to new electrical power systems on Amtrak. There also isn't really anywhere at Harrisburg to do the engine change anymore, and I can't see them doing the change at 30th street, given the train has to reverse direction here for the remainder of the trip to New York.
I would like to see mainline steam too, but we have to come up with a practical plan that overcomes those things.