If you sell large quantities of items, for instance 300 locomotives from your storage, it might happen that not all gold is added instantly into your gold balance.
You will see the correct number coming up in the gold balance, only to decrease to a lower number. When this happens, simply reload the game and the correct number of gold will be displayed again after the game loads up.
To know what happens exactly, here is the explanation :
When you sell 300 locomotives for instance, you will see before clicking on sell, the number of gold you gain for selling these. When you click on sell, these informations will be sent to our servers, in bulks. The client (the game you play) will display the number of gold you should have right away. Our servers get the first bulk of the informations right after the selling, but not all the informations, since the synchronisation works in bulks. Hence the server replies, and the game will change the gold counter to represent what actually happened on the server. In the next bulk, when the game synchronizes again, the rest of the selling process takes place, and the rest of the gold will be added to your balance.
The synchronisation takes a bit of time, but a simple refresh will do the trick and you will see the correct number.