Larry,
Check out the FreeRails forum:
http://www.freerails.com/
Someone over there may have already done this.
I usually get my batteries from All-Battery:
http://www.all-battery.com/index.aspx
but I use 2000Mah NiMh and 2200 LiPo batteries in my engines, leaning more towards the NiMh these days, mainly for standardization. You'll also need a charger, plus any on/off switches and a charging jack if you want to charge it inside the engine.
Depending on the gearing, 9.6v can run just about anything at a good speed. I get over 2 hours run time and it takes that long to recharge them (DON'T fast charge!!!)
If you don't think you'll need 2+ hours run time a smaller battery should do you well, and be cheaper too. I've found All-Battery also sells on Amazon and/or Ebay and is usually cheaper and free shipping. They give sizes for their batteries so you can make a mockup of the battery and see if it fits before you buy.