There are three main types of batteries that are of interest to us in the Disney monorail:

alkaline - these are likely the first to be used, off-the-shelf, inexpensive, disposable AA batteries.  The nominal voltage is 1.5 V but that drops quickly in a high current application like ours (500 mA).  Although inexpensive to purchase the frequent replacing gets costly in the long run, which brings us to:

NiMH - Nickel Metal Hydride is the most common type of AA rechargeable battery.  Selling for $5 to $15 for a 4-pack these cost more than disposable batteries to start with but since they can be recharged hundreds (or thousands) of times the long term cost is much better than disposable batteries.  The nominal voltage is 1.2 V but these batteries maintain the voltage under load better than alkaline and after a short time NiMH will have a higher operating voltage than alkaline.  
2000 to 2500 mAH is common for AA size and should operate the monorail for several hours.

NiMH come in two types, standard and low self discharge.  Standard NiMH have a slightly higher total capacity but  lose capacity at about 1% a day.  Low self discharge have a slightly lower capacity but can still have 80% after a year.  
These are probably the best batteries for our application. There are also high capacity "pro" types that may be a good choice for monorails with faster motors.

The final type of cells we might use is lithium, and I do not recommend anyone use them unless you know about batteries and electronics.