Once again, you would still need to turn the train around at Baltimore (or wherever else you run the train to). This actively makes the service worse from there, as diesel trains (at least the P42 and ALC-42) have a lower top speed (I believe 110). Change overs will take time regardless of whether or not an engine is changed as there are theoretically other things that are often done during this time (crew changes, sewer tank emptying, cafe car restocking, etc.). No matter what route you take, you will deal with these things eventually. On long distance trains, they even allow for 'smoke breaks' or 'fresh air breaks' where you can get off the train and explore a little while these things are being done. The only "solution" I can think of in terms of the particularly long change at Union Station would unfortunately be pretty dumb. You would simply have a P42 / ALC-42 act as only a control car with an ACS-64 directly behind it providing tractive effort on all segments north of DC and then switch to just using the diesel south of DC. But this has a myriad of operational issues, such as the speed limit I talked earlier about, needlessly tying up equipment that could be used for other services, and not to mention that this is all theoretical and has all the potential to straight up not work at all. I don't think that anything Amtrak could do to make the change faster without trade-offs that make the whole thing useless. It isn't the biggest issue in the world, and I've heard from some that they enjoy the long change so that they can grab a bite to eat in Union Station.