The piggy bank is part of a special offer that gives you an opportunity to buy Township Cash for real money at great prices. While the offer is on, you'll get additional Township Cash in your piggy bank every time you send off trains, planes, and helicopters. The amount of Township Cash you get varies for every means of transport you dispatch.
Once you've saved up enough Township Cash in the piggy bank, you can buy it all at once, or you can keep saving to buy it later at a better price. The maximum amount of Township Cash that can go into your piggy bank is shown in the special offer window.
The piggy bank offer does not affect the regular rewards you get for sending off trains, planes, and helicopters. Please bear in mind that this offer is only designed to give you a chance to buy some more Township Cash at a great price.
You can fill and buy the piggy bank an unlimited number of times while the special offer is on. If there's still any Township Cash left in the piggy bank when the special offer ends, you'll be able to start from the same amount the next time the offer comes up.