It’s a pun.

Belle is an old-fashioned word for a beautiful and attractive woman or one who is beautifully dressed.

https://dictionary.cambridge.org/dictionary/english/belle

Capitalism is an economic system based on the private ownership of the means of production and their use for the purpose of obtaining profit. This socioeconomic system has developed historically through several stages and is defined by a number of basic constituent elements: private property, profit motive, capital accumulation, competitive markets, commodification, wage labor, and an emphasis on innovation and economic growth.

https://en.m.wikipedia.org/wiki/Capitalism

In the context of “Why do Americans think everything has to profit?”, then the point is that the train is considered only for the profit it can make, and not for the environmental etc benefits. This is a result of the market economy as you rightly state (and private ownership of transportation).

Because that’s the foundation and definition of capitalism. The market will provide (as long as there’s profit to be made).

Not saying it’s right though.