This bug appeared after the most recent update that added San Francisco (Gamma13). After you loose a regular game you get the option to continue in endless mode. You continue in endless and sometimes the game decides that 63 (or whatever, something less than the top efficiency score) is the max population and 'ends' endless mode. Among the choices that come up is the option to continue in endless mode. Before the update, the menu disappears and the game UI reappears and everything continues (you always eventually to the next target score and get a new train and second choice). After the update, when the menu disappears, the game UI does not reappear, just the map: no clock, no lines and trains at the bottom, and no left arrow in the top left, when you get the next target score you get your train and second choice but without the UI there is no way to add them to the map. hitting the escape key gets you to a menu.