I use Affinity Photo but basically, I converted it to Black & White with a B&W adjustment layer. This removed all the yellowing from aging.
Next I used the paint brush tool to paint over everything that didn't look like it was the original painting, setting the color of the paint brush to whatever white resulted from converting it to black and white. This is easy, but tedious. Use the bracket keys to adjust the size of your brush as needed.

On lines, such as the lines on the box cars, if they were missing, incomplete, or painted over when I was painting white, I set the paint brush to one pixel, and set the color close as I could to the lines (3D3D3dHex, in my case). When repainting missing lines and such, in Affinity if you click on the start of a line, move the curser to the end of the line, Hold SHIFT and click again, it will draw a straight line between those two points. PS does this but I don't recall how. Also, you will have 2 colors a white for BG and the gray for the lines. Shift between them with the X key, and for the lines, adjust the opacity to get the shade you want.

For some things, like the smoke stack with the Indian blown up in the air, I used the clone stamp tool to fix the missing parts (smoke). This takes a bit more skill, so learn how to use the clone stamp. It's not hard but many don't know you can rotate things with the arrow keys. It's a tool used all the time, so good idea to learn it.

When I was all done, the black lines looked a bit faded so I added a B&W adjustment layer, and set the blend mode to LINEAR BURN and dropped the opacity of the layer until it looked good to me, which in this case was 50%. PS has the same stuff I recall, so it's the same, or very similar.

The steps in PS should be about the same. These were very basic steps with an editor, and you need to know them anyway. I suggest using Youtube to look up how to use your editor. The hardest part for me was determine what was pixel damage, and what was shading from your Dad. I also noticed after I uploaded it I missed a few dark spots in the background, but I'd already spent a lot of time fussing with the details.

Anyhow, this is a good one to learn some of the most basic editing tools, specifically the Brush, Clone Stamp and Adjustment Layers with Blend Modes. Note, when I say easy, I've been editing photos for many years. Just using layers can take a while to learn, so easy is a relative term. Don't get discouraged, learning how things work and what you can do is a ton of fun, although frustrating at times. If you get stuck, ask here, many will love to help, and remember, YouTube is always there with a billion tutorials.