Following Whatlep's excellent review, I bought some Li batteries.
I have two of the blue wrapped type 6800MAH, and one in a hard black case, 3800MAH.
The blues came with 3 pin chargers, the black with a 2 pin and adapter.
The blues have a female trailing socket, permanently live, and a male trailing plug, switched.
I leave the plug unused, and have since cut it off and used it in the loco wiring.
The socket feeds via a plug to the loco wiring, switch and charge socket.
Both work well, perhaps a little underpowered for double bogie motors.
The black cased battery has a switch and socket. It also has a 5V USB socket, as yet untried.
I plug the loco wiring into the 12V socket, and remove it and the battery for charging.
I recently bought a larger blue wrapped battery at 9800 MAH, and found both leads are switched.
This means the tiny LED is permanently lit when the loco switch is off.
There was no way to make the switch accessible without cutting a large hole in the body work.
It came with a 2 pin charger and adapter, having asked the seller for a UK 3 pin charger.
Chargers all have two lights. The red is on when plugged into either the battery, or the mains, or both.
The green is on while charging and goes off to indicate full charge.
All are rated at 350Ma.
Not much more to say, except that 12.6V is rather low for double 24V motored locos.
I've recently been absolutely staggered at the price of 21.5V 2100MAH Li Ion packs. eg USAT type.
Correction:- for USAT I meant Aristrocraft.