I'm still working my way through using Tamiya Panel Line Accent Clor for model railroad weathering. As I've said, this has been mainly a product for plastic modelers of aircraft and spaceships, and the typical instructions for use I've seen on, the web aren't always practical for railroad models. Inserting it into fine panel lines and using capillary action to spread it, and swabbing away overflows, isn't as practical. Instead, I've taken to just swabbing it over the whole model area with a brush, letting it settle in and dry, and just soaking up obvious runs with a piece of paper towel, but otherwise letting it do its thing.
As one YouTuber said, the dark brown shade seems to work best on orange or yellow cars. It also works best on cars with fine cast detail, like riveted sides and Youngstown doors. The effect is often very subtle. Here's an Athearn EJ&E box:
Its effect was mostly to highlight the frame stiles and edges of the Superior door. (I had already done a light airbrushing of dust around the lower body and trucks.) Since the sides are welded, there wasn't a whole lot of other detail to bring out.
Here's an Accurail Minneapolis & St Louis boxcar with better possibilities for the accent color to work:
Even though the car isn't yellow or orange, the dark brown shade settled in very well in the Youngstown door, along its edges and door tracks, and around the ladders and grabs. Here and there it also darkened some of the flat areas of the sides.
Now we come to one of the best outcomes so far, an Accurail Katy 50-foot box. I have a "before" shot, made some years ago, below. It's fuzzy, but it gets the point across:
And here's the "after" shot:
The paint takes a lot of the shine off the original model, and it settles into the door recesses, along the edges and door tracks, around the rivets, and around the ladders and grabs. One concern I have about factory painted models, especially in light colors, is that the paint can look too uniform. The laws of optics don't scale down. This helps take away that effect.
Finally, here's an Atlas Katy GP7 in as-delivered red:
Although the effect is subtle, and it's a little darker in person than in the photo, the accent color did a good job of highlighting the radiator air intakes and the lovers on the hood doors.
There are wide variations in freight car weathering, and there will never be a single magic bullet for modeling it. I find this does an effective job in certain specific cases of light weathering. Other Tamiya shades may also have a slightly different effect. It goes on fairly quickly, dries in about half an hour, and can be used to age a good quantity of cars in a fairly short time.