[daddylonglegs]

IIRC very few public transport systems make a profit on running costs - never mind capital costs. The rest operate as money pits.

Useful systems have huge positive externalities that can't be captured by charging passengers at a rate that reflects those benefits (I'm not sure if this is due to a free-loader effect, irrationality on the passengers part or something else).

I think the only systems that have profited on scale commensurate with their benefits are those that were a real estate play, eg.: the Metropolitan Line[1] in London, Los Angeles trams, Hong Kong subways. I might be wrong about these examples, either due to misremembering or to falling to a just-so story.

[1] https://en.wikipedia.org/wiki/Metro-land

Edit: formatting.

Edit 2: The point, I think, is that if you rely on direct profits for the transport system you give up on huge benefits and accept huge costs across society.