How to make a prop dynamic receive correct shadow as it moves?

Guys,
I'm trying to do a simple 'train through a dark tunnel' feature (like the ones in Well). I've got the train moving happily around a loop (prop_dynamic parented to a func_tracktrain). The problem I'm having is the the prop (bomb_cart.mdl in this case) is always light as it appears in its initial position. What I want to happen is that it comes out of a dark tunnel, gets brighter as it passes the player standing on the platform, then as it disappears down the tunnel, it gets darker again. Looking closely at the trains in Well, when they are in the tunnels either side of the map, they are darker. Once they come out into the sunlight, they brighten up. Therefore it does seem to be possible to achieve the effect I want.
Can anyone please suggest what I'm missing to make this work? Cheers
EDIT: The screenshots show the train as it first appear in the dark tunnel, yet it is still brightly lit. The lighting stays constant all the way along the tunnel.

I think you're referring to this from the developer commentary notes:

We build our 3d skyboxes at 1/16th scale to reduce the memory used by the large spaces in them. This means we have to get a little tricky when dealing with trains moving between the skybox and the player space. There are actually two versions of each moving train; A player-scaled one for the actual gameplay space, and a tiny one out in the skybox. The small repair sheds on either side of the middle building disguise the point at which we swap between the two trains.

Click to expand...

AFAICT the transition is purely so they can switch between a skybox (1/16th) model and the real model. The shed in Well just outside the playable area where you lose sight of the train briefly seems to be where this happens. Once the train comes out the the shed, there is still a visible transition of its lighting state. It's this transition I'm trying to reproduce. Perhaps I should look at a decompiled version of well...

Thanks for your comments. Now for some follow up:

I changed the model from the bomb cart to the same train engine as used in Well. At first this appeared completely black as it went down the tunnel, rather than fully lit like the bomb cart. After tweaking things, I eventually fixed it (for the train model) by tweaking the lighting values (I'd initially set the falloff distances too low). Now the train (func_tracktrain + prop_dynamic) travels down the tunnel out of the darkness, gets brighter as it comes under the lights, and then gets darker again as it moves into the shadows. Perfect!

If I change the prop_dynamic to use the bomb cart model (without changing anything else at all), then the cart stays fully lit at al times. It seems there might be something in the cart's material that behaves differently from the train's.
That does fit with the post above somewhat, so I'll try experimenting tonight with the bomb cart as a prop_physics rather than a prop_dynamic, and see how it goes.

For reference, all my lights are static (a mixture of light and light_spot entities).