Can you point to common examples of single train service between points A and B where there isn't a monopoly?

Big airlines have multiple hub and spoke networks, like the internet. If AAA-BBB can't be operated due to some disturbance along the way (not at AAA or BBB), they can just route traffic AAA-xxx-BBB. Many airline major city pairs also have high frequency, something Amtrak generally lacks outside the the NE Corridor.

Acela trains are sold as premium service. I'm not surprised Amtrak sought the surcharge.