The design spec calls for the trains to be capable of at least 250 mph. However, maximum speed in densely populated areas such as between San Francisco and San Jose will require the speed to be capped at 125 mph.

Much of the planned route is covered with foothills. The hills either require extensive demolition or lots of twists and turns in the track. Integrating a new light rail system with existing rail or road rights of way requires a lot of turns, too. Turns limit potential maximum speeds.

When you also factor in the time spent slowing down, stopping at stations, and speeding back up again, a 4 hour trip really is a best case scenario. The marketing talks about 2 hours from San Francisco to Los Angeles, but that's unrealistic.

Is there enough of a market for the software to support your current employer and a new competitor? Maybe your employer's software is less than stellar because that's what they can afford to produce based on what the market is willing to pay.

Are any of your friends sales and marketing experts? Even if you make the most wonderful product in the world, you still need to get customers interested and close deals.

Can you and your friends afford to self-fund a startup for two years? If not, you'll need to find investors, and investors will generally want you to hand over most of the potential profits in exchange for them floating you the cash.

You could end up working for two years with limited income, no job security, and only a small chance of turning a profit.