Hard but satisfying and beautifully executed. Deceptively simple, great progression and challenges. Really imparts something of the furious logistical chaos of underground rail (and some others, elevated, trams and shinkansen) across the world - with their own flavour and geography!
Cheaper in Google play ~£1.20, but available across multiple platforms.
That does sound like a good price on Google Play. Mostly it reminds me of people building Metro systems on growing Minecraft servers :)
https://map.civmc.tk/