I've been building a mobile app for quick and convenient public transport departure lookup in Switzerland[0], which is doable because there's a free, well-documented public API[1]. Since it's fairly popular, I thought about extending it to other countries, but as mentioned above, the API landscape looks super heterogenous and spotty.
[0]: http://billhillapps.com/billhill/quickov.html [1]: http://transport.opendata.ch/
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https://twitter.com/_mades/status/1075228269845336064
What this seems to boil down to is a side channel for certain partners with looser technical controls but tighter monitoring of misuse, and no evidence that that tradeoff actually resulted in any misuse that couldn't have occurred through normal data access channels.