Hi, don't take my advice as pro's advice because i'm not, but i think that you should first crop the image to have more of your subject, better than i did but something like that... Then because there is yellow not only on the train but also on the trees and the grass, you should not desaturated the colors, just down a bit of saturation but keep those and accentuate the yellow. If you have a color mixer in your soft you can do it. For me it will be the best way to go.
Hi, don't take my advice as pro's advice because i'm not, but i think that you should first crop the image to have more of your subject, better than i did but something like that... Then because there is yellow not only on the train but also on the trees and the grass, you should not desaturated the colors, just down a bit of saturation but keep those and accentuate the yellow. If you have a color mixer in your soft you can do it. For me it will be the best way to go.
Hope i helped a bit.