Had the same problem. It isn’t 100% dark, but visibility range drops to about 10m, and everything is at best dark grey. Even the headlamp only raises a few surfaces to about 10% above black, but a lot of surfaces stay black.
Solution:
Switch to DirectX 9 and Restart the computer. Everything appeared normally once I tried that. I messed with all of the other settings first (brightness, gamma, contrast, essentially all of the radio buttons). It is possible that Restarting by itself worked, but I don’t think that’s the case here.
I did have an excessively dark mission earlier, but muddled through it. Darkness in Last Train is murder, because you can’t see the sides of the train, cant’ see gaps to jump over (black on black), when up top you get nowhere near enough warning of the bars coming, since the lights shows up with 5ms to react instead of 3s.