I think I'd try India ink washes at this point. Use about half a paper cup of alcohol and a few drops of India ink. Brush that on, let it dry, and see how it looks. If it's still not black enough, just keep adding coats until it gets to where you want it. Willis's trestle would be about "middle-aged" in terms of color, with some of the black creosote weathering off and showing the timber color underneath. Inda ink washes should give you a similar look.