Recommend a market structure for high speed railroad in the US

First, we need to make sure you are familiar with the market structures mentioned in the question. Based on a review of those structures, I believe we can eliminate monopolistic competition, because a market structure with many sellers is almost surely not going to work. Monopolistic competition is best for branded products, like laundry soap and toothpaste. So we're left with a monopoly (one seller) or an oligopoly (few sellers). Both of those structures exist because of the large amount of investment capital it takes to start a business like an airline. A small local retail store (like a beauty salon, for example) takes much less investment capital. A monopoly example would be the US postal service, or perhaps the US Interstate highway system. The benefit to customers is that there is one system and it is unifed and consistently available to everyone. The cost to consumers is that we must pay taxes to support the system and there is no competition to lower costs. An oligopoly would be a limited number of sellers - think of the passenger airline business. That works better than a monopoly for consumers because there is price compeition. That is the same reason it doesn't work as well for sellers. It may not be the best option if safety is sacrificed to achiever lower prices, either. However, I think that the infrastructure investment for airlines is different that for a railroad. One system of railroad tracks is likely to work well, just as one highway system works well. So i would recommend a government sponsored monopoly, similar to the US Postal Service.