They have another article about a more conventional tunnel idea that addresses that issue. They were well aware of the issue. The only way to make this work at that time would have been something like Brunel's atmospheric railway. That was plagued by issues when in the open, goodness knows what it would have been like to run underground for such a long distance.

https://www.theengineer.co.uk/content/in-depth/this-week-in-...