Not that huge. This thing seems to have been scaled for narrowboats, like the old British canal system. Those have some tunnels, just barely big enough for the boats. Here's the longest canal tunnel in the UK.[1]

That was built in 1803. The project in the article was proposed in 1907, which was way late to be getting into narrowboat canals. Railroads, both ordinary and cogwheel, were working just fine in Swizerland by then.

[1] https://www.youtube.com/watch?v=9ETwZuu9yZ0