Everyone should watch DoNotEat's Loop video. It's not short, or polite, or fair. But it's right.
The short version is that trains work fine and anything that's just "like trains, but" is a bullshit distraction.
This guy is the straight man and architect of Well There's Your Problem, an engineering disaster podcast... with slides. They did the Las Vegas Hyperloop a while back. So if the 40-minute video about how tunnels are hard and trains work isn't long enough, vulgar enough, or communist enough for you, there you go.
... though watching the first video again, I'm wondering if you could dig faster by cutting around and behind each segment. Like if what got hauled out of the tunnel wasn't gravel and dirt, but giant cheesecake slices of rock.