I'm not sure if they have fixed this in the latest update, but if you build a boardwalk tile and the little title icon is floating and ready for you to collect, leave it alone. Have your friends tap on it instead as part of their daily activity in your city. You'll accumulate a # of handshakes for your boardwalk, and when you finally collect them, you'll receive multiple boardwalk tiles, for the price of the original tile. We been able to get this to work with 3 at at time, but some people can get up to 8 tiles at a time. Definitely helped with rapidly expanding the boardwalk
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