The piggy bank is a special event that gives you the opportunity to buy the Westbucks at a bargain price. While the event is on, you’ll get additional Westbucks in your piggy bank every time you send off trains, steamships, and wagons. The amount of Westbucks you get varies for every means of transport you dispatch:

Once your piggy bank is halfway full or more, you can buy the Westbucks at a bargain price. You don't have to open the piggy bank when you reach the minimum amount required. Instead, you can continue saving up. The maximum amount of Westbucks that can go into your piggy bank is shown in the special offer window.