But why make it a traditional train? Just build a lightweight train if it gets you all of the hyperloop's cost advantages without the additional costs and complexity of a tube?

On trains the cars have to be connected to each other. The structure has to bear the load of all of them at once. Not the case with hyperloop.

The reason we have to run long, heavy trains is it's unsafe to run two passenger capsules at 300kph a minute apart. I find it hard to believe regulations will permit that even when they're in a tube.

It's totally implausible that that's the primary factor.

Why? You've given no explanation.

Nor has jim-greer.

Traditional trains have wheels and engines, which are quite heavy. And with a traditional train you're trying to minimize losses due to air resistance, which is not an issue in a low pressure tunnel.

Above a certain speed wheels present huge issues, which means you need to change your drive mechanism and suspension mechanism.

Also, drag is a huge killer of efficiency and hence overall economics. Even at 500kph the drag on a train at sea level is enormous. At 1000kph it's 4 times bigger again, and you've got the sound barrier to worry about. Trying to do the same with multiple small capacity trains is astronomically worse. So, better to have reduced drag... and by now you've basically got the hyperloop.

My bet is that Elon could build the Hyperloop closer to budget than the state of California could build a traditional train. Leadership matters.

It's not clear to me that he is interested in being the one to build it. I think he wants somebody else to.

I'm sure he'd prefer someone else do it but then again he might have preferred that someone else would have conceived of and designed it -- or something similar. There's a chance, as with the leadership of Tesla, that no one else with the capability (or position/reputation) to execute will be found, and I'd be surprised if he were not heavily involved in seeing it through.

From what I understand he still sort of wants out of Tesla in the long term. SpaceX is his baby and he's only running Tesla because he feels that he has to. I suspect he isn't receptive to spreading himself out over a third project (he apparently doesn't do much work himself for SolarCity).

My bet is that people in the middle who don't get any benefit from it will prove just as obsrtuctive as they are towards any other large scale infrastructure project.