While I am at it I also need to find out about travel between Wash DC and Myrtle Beach SC. I have checked the stations and it looks like there are a couple of opportunities there as well. Any info you have is much appreciated. I will likely have a ton of questions on Sunday in chat too so I can get some help then too
Unfortunately there is no AMTRAK service between Washington, DC and Myrtle Beach. There never was direct service since at that point the tracks go nowhere near the shoreline. However, up until two years ago, there was a Thruway bus (a bus that can be reserved through AMTRAK that provides connections to or from a train) between Florence, SC and Myrtle Beach. In Florence it met both of the trains that operate on that route (the PALMETTO and the SILVER METEOR). It was cancelled due to low patronage, although I would guess that it was not too bad during the summer months on days appropriate for the beach.
Sorry I can't help you more with this. You can still get to Florence, but you would then have to rent a car there and drive the remaining 60 miles, or so, on US 501.