A safe battery power system for the track power supply for LionChief train sets?

Couple thoughts...

First, lowering the track voltage is NOT an entirely effective way to limit engine speed on LionChief/Plus locomotives.  From what I've been able to see, it seems that only the fastest speed step can be eliminated by reducing track voltage, all other speeds can be reached with as little as 7.1VAC on the track.  the max speed step attainable at any given voltage will vary based on how much of a load the engine is pulling, for example, the next to fastest speed can be reached with just 7 volts on the track running just the engine, however with 3 or 4 cars being pulled a couple less steps will be available.  The more of a load, the more voltage is needed to have all the speeds available.  

In my testing, 12VAC provided full function of all speeds, even with a substantial load, however the smoke unit needs 14-16VAC to work well.  I have not tested the engines on varying levels of DC, but I expect a slightly higher DC voltage will be needed for the same performance...  I expect about 1.4 times higher DC input for the same results. I can test this sometime next week if needed. 

There are a couple guys that run battery power RC that could recommend good battery choices to place inside the tender or perhaps trailing boxcar.  Li-ion batteries are typically used for light weight , compact size and adequate run time.  

Over all, if the intention is to power the track from battery, I'm not sure that it is any safer than using the wall pack that is included with LC sets.  The breaker in the wall pack is quite responsive and will trip long before any harm comes to the trains or any child playing with them.  If the main purpose is simply to use a lower voltage on the track you could try a computer power supply to run the track at 12VDC, but it would probably be simpler just to use a small post-war AC transformer with the throttle turned down.  

JGL

Edit:  for speed control, a more effective method, though a little complex, is to modify the remote.  One can add physical stops to prevent the knob from being turned too far, or modify some parts internally that would electronically limit the speeds.  I haven't tested the electronic method, but I do not believe it is exceptionally complex based on modifications I have done to the remotes.  

Thanks for the thoughts. I am just considering for the kids sets like Thomas not the fancier engines with smoke or the LionChief + engines.

For a few years I have been setting up several basic LionChief sets at shows for the kids to run. I hook up a common power pack for most all the tracks these engines are on.  I set all the LionChief remote controls to full speed and just adjust the power pack speed control lever till all the LionChief engines are doing a reasonable speed , not to fast or slow and they all work fine on the slower speeds.

I was just thinking of a battery supply and not having to worry about any cords for a kids layout to run on batteries instead of having to worry about a plug in the wall with small children.

The other option of course is to instead use one of the Lionel battery operated sets. That perhaps would be easier but a much smaller toy scale or the G scale battery sets.