Board Thread:Questions and Answers/@comment-27095242-20151102192131/@comment-15636815-20151103025610

Rolf's answer is correct. To add a tiny bit of clarity to his answer, the game tracks the number of slots and the number of rails separately and independently. They really have no relation to each other at all, unless you consider the free slot received when purchasing a rail to be a relationship. (I don't.)

Therefore, there is no limit to the number of train slots per raila ratio that is irrelevant in our game. Only two things limit the number of train slots you may purchase:
 * your level, as Rolf said
 * the number of gems you are willing to spend when Bonus Train Slots appear in the Special Offer