You ARE aware that "The Waltons" was originally filmed in the 1970s and very early 1980s, correct? That these are reruns of episodes filmed 35-plus years ago?
The episode to which you refer is apparently the two-hour season 9 opener, "The Outrage", which originally aired Nov. 20, 1980:
http://www.the-waltons.com/season9.html
Nearly everything one sees in "The Waltons" was filmed on Stage 26 of the Warner Brothers Burbank, Ca. studios, with a few outside shots filmed at various locations frequently used by Hollywood studios, such as the Golden Oak Ranch, a Disney-owned property north of Los Angeles that features a very fake-looking covered bridge.
Having said that, the producers of "The Waltons" were known to have shot background locations in the central Virginia heartlands. I'm certain that they might have seized the opportunity to do a little filming if a steamer were about. However, 4501 was primarily used in Tennessee, Alabama, Georgia, and Mississippi in the late 1970s with no trips to Virginia. Meanwhile, SR 722 (then also painted green) was used on trips out of Alexandria in March 1980 before failing on March 29 and being replaced by diesels on a March 30 trip to Warrenton. More trips with 722 were run in Va. and N.C. in August 1980 as well. And, of course, all of the filming could have been done at the Tennessee Valley RR Museum anyway, the most likely scenario........
On one side note, the show was set in the fictional Waltons Mountain, which in real life was Schuyler, Va. One frequent plot of the show was going over to "the Rockfish post office," which was supposed to be a general store trackside to the Southern Railway. Rockfish does exist, and also the supposed "post office," a tiny prefabricated building supposedly "dropped off" by the US Post Office in 1910. The store burned down around 1970, apparently. During one of the last NS steam runs in 1993 or 1994, I shot N&W 611 pounding over the trestle there and past this quaint little shack, which at the time still had a rocking chair and vintage Coke machine on the porch, as if I'd dragged them there to pose them!
https://www.facebook.com/pages/Rockfish ... 6132825864
Webcam pointed at the tracks for railfans:
http://www.wunderground.com/webcams/nobyter/2/show.html