Then comes a time where there are no new markets to exploit, so you build a a new app or a continuation of the old one, a sequel. At this time you also lower the price to 0.99 just to make sure you have the most exposure possible. Only now you have the advantage of past and present publicity and if rightly timed, momentum... And then off course you repeat the formula depending on your market and the viability of your app.
"Then comes a time where there are no new markets to exploit"
The crazy thing with iOS development is that there are 250k new users every day who are always looking for a few apps to put on their new phone, which is how a game like Angry Birds can stay high in the charts for months.
The cool thing is that now that you've been the developer of Trainyard, when you do reach a point where you should be releasing a new game, you have gotten so much credibility it isn't even funny. And off course that translate to more immediate sales at the beginning and a big chunk of extra steady clash flow at the end.