Hi David,
I do think us old dogs can learn new tricks. I keep learning new stuff on the guitar.
You probably know this stuff already, but just in case, let me offer some encouragement.
First, if you were able to get a simple version of "Freight Train" down in a week, I think you are doing very well.
When you refer to a syncopated version of "Freight Train," I wonder if you are referring to finger picking the song with a Travis picking pattern. If that is the case, then I understand that learning the Travis picking pattern is a much tougher task.
I have done a couple of video tutorials on how to play the Travis pick that might be helpful to you. Going through these should not interfere with your lessons; my videos could just be a supplementary thing that might help your brain. They're free...
This is a lesson on using the Travis picking pattern to play "Clay Pigeons" by Blaze Foley and as played by John Prine. This is a good lesson to start from because the tempo for Clay Pigeons is fairly slow and so it gives you a chance to get these patterns down without requiring much speed.
This is a lesson at a faster tempo for playing Bob Dylan's "Don't Think Twice" using the Travis pick.
The latest research on slow wave sleep seems to suggest that when our brains go into this mode of sleep, our brains are wiping away unimportant memories from the day while also helping our brains organize and consolidate important stuff we have learned. If you keep working on learning a new task day after day, your brain will reward your tenacity. Every time your brain goes into slow wave sleep, your brain is again working on organizing that information to help you make sense of stuff. Over time our brains learn in layers, so repetition really helps and is essential.
I find this information about slow wave sleep helpful to take the pressure off of myself when I am learning something new. (I keep learning new stuff and I am age 73. I have been playing actively without losing my calluses since I was age 16.) I just work on stuff for a while until I'm almost to the point where I can't stand it anymore, and then I let it be and come back the next day and do it again. And then keep repeating this approach until after two or three weeks I have integrated the new technique into my brain and my fingers.
I hope this helps a little.
- Glenn