What they're saying is that there will be just as many cars on the road as before and therefore not reducing the environmental impact of the road.
I imagined it to be something like this:
Old:
- Road: 1000 users
- Train tunnel: 1000 users
New:
- Road: 1000 users
- Train: 2000 users
Although the train is more environmentally friendly than the road, I think the article is saying the impact of adding it is just that more people make the trip rather than the same number of people doing it in a more environmentally friendly way. This is basically the concept of induced demand i.e. build it and they will come.
Isn't the environmental impact of 1000 people traveling from point A to point B via rail far preferable to than 1000 people making the same journey by car? The quote above seems to imply that rail provides no appreciable environmental benefit, I'm finding that hard to believe.