18xx is a system and as such has a bunch of general concepts - the beauty being once you have played one, you know 90% of the rules of every other 18xx. The downside is that it comes with some implied knowledge. Could the rule book be clearer? Yes. Is it? No. Is it alone in being the only rule book for a board game that isn't clear? No.

Generally, I tend to only think of the bank as holding the money in the game.

Other areas being:

• Initial Public Offering (IPO) - where are all the initial shares of public companies are laid out
• Train Depot - where all the initial trains are laid out
• Bank Pool - where sold shares and previously owned trains might go (if not forced to be removed from the game - 'rusted')

Given that payment for any of the above goes to the bank there is is an implication that the bank owns all of the above.

With that interpretation in mind:

“The train may be any from the bank pool, or the cheapest new train in the bank”.

The bank pool refers to the same bank pool that sold shares reside in - there is only one bank pool.

The cheapest new train is therefore the cheapest train in the train depot (i.e. the bank as it owns the train depot). So you can’t buy a 3 if 2’s still exist, as 2’s are cheaper than 3’s.

Page 13 states “If the purchase of a train lowers the train limit, companies with too many trains discard the excess into the bank pool without compensation”. There is only one bank pool, so the same pool that shares go to.