You're not going to find a lot of businesses opened on Sundays in South Central PA, including any of the shops listed in this thread. Looking at the hours, all the shops are closed by 5 on Fridays. It's amazing how many train shops were once in this area even five years ago and now its down to a small handful.
Mainline Hobby in Blue Ridge Summit is open on Sunday afternoons. That's a little over an hour south off US 15 southwest of Gettysburg.