Battery Sourcing

Check ebay too. they have many many batts that may fit perfectly for a few $$

This one is 2.4 V, 800 mah, $2.63 SHIPPED!

I am unsure if it matters for the mth locomotives, and I am unsure what is shipped new, but if you can find them for a fair price I would recommend looking for NiMh batteries that state "Low Self-Discharge"  These will hold about 70% charge after a year storage (real world tested with the Duracell, name brand, ones.)  May not be an issue to have a dead battery when you first apply power, But I've heard enough on the PS1's to avoid it if possible.  As a note, NiMh batteries that are not specifically marked as "pre-charged" or "Low self-discharge" will drain on the shelf much quicker than NiCad's.  Usually 3-5 days in my experience.  

Note 2:  I use only Duracell "pre-charged" rechargeable in my camera equipment and they are my top choice for rechargeable batteries.  I'v tested Rayovac and they seem almost as good.  other brands have been hit or miss with how long they hold a charge and with how many charge cycles they last.  Energizer falls to the bottom of the list, as I have never seen a set of their rechargeable batteries that has lasted more than 9 months without at least one battery failing.  

For me, having never spent more than $400 on a locomotive, and much less on most, I see the merit of finding cheap batteries, however with most of the engines out there way north of $500, I don't know that I see the value in taking a chance on some of the cheap ones.  I've seen $2000+ camera bodies destroyed because folks wanted to save a few bucks on a battery.  Anything I put in my camera will always say Genuine Nikon for that reason, and only Duracell in my equipment.  

Lithium Polymer batteries have excellent power to weight ratios.  But anything that says LiPo also has potential for problems. Knock-off LiPo even more so. IMO. You find these batteries in most laptop computer.  They are also very popular in the model airplane hobby. 

Yes, Li-Po are great ...BUT, charging them is not the same as ni cads nor NiMH...Be extra careful...if you use the wrong charger, a guaranteed fire/explosion will happen...FYI...