I found another way that is pretty similar to Buzz's method and both work great. First you select the area of your curved shape from a good image of the locomotive. Then you paste it and smooth the edges while removing any handrails or other objects you don't want in the image. Next, select the area without copying the image, drag it over your alpha layer, and color in the area using the overlay paint option. To make it all kinda make sense here is a screenshot (I hope it's not too confusing lol) :
bandicam 2020-01-07 21-57-38-125.jpg
Edit: The edges are real jagged/pixel like on the curved section, so it'll take some work to get it smoothed out. Actually, if you use Buzz's method combined with this one, you could outline the shape from the image using the tool Buzz suggested. That is what I have done before, since it will give you a nice smooth edge to your curve.
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