The best word of advice I can give about batteries is to check how much space you have. You can always try to find a battery pack with as large a Mah rating as you want, even though you may not need it, as long as it fits.
The other consideration is how are you going to charge it, in the engine/tender or pull it out and charge it on the bench. I'm trying to configure mine so they can all be charged in the tender as I'm doing steam only right now. My engines with NiMh batteries have a charging jack module that needs a round mounting hole and enough space to mount it.
On my engines with LiPo batteries I'm having to make a mounting plate for the charging connector and cut a rectangular hole and drill 2 holes for mounting screws somewhere on the tender, or open the tender up to gain access to the connector.
A couple of my engines (Williams N&W J for one) have enough space that a 1.3"x1.3"x3.9" NiMh battery pack had plenty of room whereas the MTH Railking Imperial 0-6-0 had just enough space for a .91"x1.38"x4.13" LiPo battery pack.
Others could have used the NiMh but I chose to go with the LiPo because of the higher voltage at startup (less internal resistance).
Honestly, I wouldn't get too concerned with all the talk about Mah ratings, unless you plan on running non-stop for 12 hours at a time or plan on pulling a 20-car or more train. I plan on using at least 1000Mah, but for something like the 0-6-0, where it's used continuously for maybe 1/2 hour at a time, I feel even a battery with a smaller Mah rating would suffice.
I will say that I'm not using sound at present, only powering the DC can motor, headlight, and backup light (both Evans LEDs), but I'm getting a lot of run time on all the battery packs (2000-2200 Mah). Since April/May I've only charged batteries once and run trains at least once/daily. I've been getting my battery packs from ALL-BATTERY.com, very fast service from coast-to-coast and good prices.
