Steam and Age of Steam provide a finite set of tiles, and the rules explicitly mention that if a certain type of tile runs out then it is effectively unavailable anymore -- unless a tile comes back after being replaced by another one.
This rule seems quite artificial, as in real life there shouldn't be any problems creating any kind of track as the train line is being built. The original designer Martin Wallace himself wrote that this constraint was artificial and tried to create a "friendly" set of tiles.
I do agree that it is currently part of the game and I sometimes use it myself to get rid of some particular tiles so that my opponents may not use them.
Some expansion maps (e.g., Sun) were designed with the limited set of tracks in mind, as an important aspect of that map.
However for most maps I sometimes wonder if we could just forget this limitation, provided that there's a way to create missing tiles as needed.
So what do you think? What would be the pros and cons of relaxing this rule. Also, how would you implement an "infinite" supply of tiles?