Any tricks to make the tunnel dark?

In real life, tunnel are usually very dark
Although fog seems to be the best way of recreating the darkness in tunnel (Without losing the tunnel texture itself for the headlight), it creates a problem where fog also applies to train interior.
Is there any tricks that I can use to avoid this? Thanks! 

I am pretty sure implementing this as a new OpenBVE feature won't be viable as I can't really think of a way to accurately define the section that does not need fog

This is something I've been thinking about too.

I'm building a UK route, the Isle of Wight route, it features a 400m long tunnel and I'm currently using fog to created the effect of darkness which looks good within the tunnel but it's the transition from light to dark and then back to light that I can't get right.

All my stuff used a combination of .brightness in the csv file and Photoshop grad textures on the portal object. Once inside the bore, a very dark wall serves throughout the tunnel interior.  Similar techniques also applies to the track items.  I've NEVER bothered with .fog

The problem is I want to get headlight working.
Inside the tunnel, it should only be plain darkness (Or at least VERY dark).
Darkening the texture could cause the tunnel texture to be washed out, and is quite visible on high gamma monitor/with headlight.

(BVEWesternRegion's route also has the same problem where it looks realistically dark in the cab, but fog covers up the train interior)

Unfortunately I don't think there's any way to completely avoid this. I either have to choose to have a better experience in the cab or a better experience in exterior view.

Thanks for the suggestion though!

Adding a light fog that becomes visible in something like 40-50m from the camera is actually fine, as long as it's just a light hue of darkness. Headlights can be implemented by adding semi-transparent beam objects with additive color blend.

Honestly Openbve really needs to introduce per-pixel lighting somedays though. This will enable proper darkness, headlights and shadows.