The wheels are finally turning for Elon Musk's Hyperloop.

The nearly 800-mile-per-hour transportation tube concept Musk introduced in 2013 reached a significant milestone Wednesday, when Los Angeles-based startup Hyperloop Transportation Technologies announced it had filed permits to build a five-mile test track in Quay Valley, California. The company's CEO, Dirk Ahlborn, says he expects construction to begin later this year.

HTT faces competition to create the first Hyperloop prototype from fellow Los Angeles-based startup Hyperloop Technologies. But before either company can build a working Hyperloop system, plenty of questions remain. "As a science fiction idea, it clearly works, but as a business, I'm not sure," says John Hansman, a professor of aeronautics and astronautics at MIT. "In order to do it, you'll have to do it safely, and that's what will be expensive."

Here are five questions that need to be answered before the Hyperloop can become a reality.

1. How much will it really cost?

While Musk has estimated the Hyperloop might cost upwards of $6 billion to construct, Ahlborn has publicly stated he expects the project to cost $16 billion. Meanwhile, Michael Anderson, an associate professor of agricultural and resource economics at the University of California, Berkeley, estimates $100 billion is more accurate. Ahlborn has stated he has more than 400 investors willing to buy into the project--and says the Hyperloop could potentially sell energy by producing more power than it needs--but even at the lowest possible estimate, it's unclear whether the project would be able to attract enough funding to get off the ground.

2. How would passengers be evacuated in an emergency?

Because the Hyperloop's capsules would travel within a depressurized tube, getting passengers out between stations would be tricky, to say the least. "If a vehicle becomes disabled and stops, you can't get out of it without basically raising the pressure in the tube or having a pressure suit," Hansman says. "You can design ways to do it, but it's going to be complicated and expensive."

3. Can you reliably prevent passenger injuries?

Though Hansman says the human body can comfortably withstand the acceleration levels that would come with traveling in the Hyperloop, creating a safe environment for passengers would require designing extremely smooth inner walls for the tube without any rough edges. Why? When traveling at speeds approaching 800 miles per hour, the tiniest irregularity could cause significant damage. "It would create a lot of sideways or up and down forces that would break the capsule," Hansman says.

4. Is it possible to cut through all the red tape?

Setting aside the physics, the bureaucracy of acquiring land rights between large cities like Los Angeles and San Francisco and dealing with all the legal issues around large-scale infrastructure construction could be significant hurdles, regardless of who builds a prototype system.

5. Could enough people afford tickets to make the system economical?

Traveling from Los Angeles to San Francisco in under 30 minutes sounds great, but would the cost per ticket be low enough to attract passengers who are accustomed to flying between these cities?

Next week, Musk's SpaceX is hosting a capsule design competition where high school and college students will present designs in hopes of winning a $50,000 prize and the opportunity to work with HTT competitor Hyperloop Technologies.