Faster Travel

I read in a Star Trek book back in the day that you travel faster in a straight line compared to an arc. Yet Jets, our fastest way of traveling, fly in an arc. Imagine if we had underground tunnel systems, with high speed trains taking us straight to a point across the globe through the core. If nothing else, they could take us to a point where Magma is all that is around. I propose a high density, heat treated metal to build submarines to travel through the magma. Tickets would be expensive, but imagine the profits for those supplying the way for people needing to get somewhere quick, and even the tourists! I expect the subs would be quite large, and the propulsion system wouldn't have to be a propeller, but something better. Maybe something like a waterwheel, so it pushes you forward, but also uses the energy to power said submarine.