This battery hack is for the Lionel 6B only. Other models are wired differently and require a different rechargeable wiring setup.
Step 1: The Toolless Hack
I wanted something simple and off the shelf to keep a Lionel 6B charged without having to carry around a separate charger. Radio Shack sells 4.5v and 6v mini solar cells. The Lionel 6B runs on 3V. Without some simple voltage limiter it would be very easy to over charge the batteries chosen and/or destroy the Lionel itself. These units are 50 years old and fragile as is, so this needs to be idiot proof. The 2 AA 3.2v lithium iron phosphate cells are wired in series, but only one is inserted into the Lionel battery holder. The other is inserted in a AA cell holder and taped beneath the battery holder to keep it out the way. The second AA to D cell adapter is made into a dummy cell by wiring it across inside.
I settled on using a 6v mini panel from Radio Shack. $7 normal price in store.
2 of these AA sized 3.2v lithium rechargeable cells Online only. The one shown above were bought from eBay in bulk because I use them for other things.
1 of these AA battery holders.
2 of these AA to D cell adapters. Radio Shack, but online only. These are available from eBay China for less if you are willing to wait. The ones shown above are from Ebay. I already had them.
Some 2-sided foam tape to attach the AA holder to the bottom of the Lionel battery holder and to attach the solar cell to the side of the Lionel base opposite the check source.
One schottky barrier diode to prevent the batteries from discharging back into the panel in dim light. I put my diode on the battery pack side. Schottky diodes have a low forward voltage drop. The panel will put out more than 6v in direct sun, but it's output fades a bit in the heat, so to get close to 3v for each cell you will need all the panel voltage you can get. A Lionel will still operate correctly with 2.5v according to the booklet that came with the unit. The booklet recommends replacing the batteries when they get to 1.25v each. Nicads and NiMH cells operate at 1.2v each, that's why they are not a good choice for a CDV. I have operated this Lionel on 3 NiMH cells in series just to see if it was possible and it worked correctly but fully charged a 3-cell NiMH pack would have over 4.3v Undercharging a LiFePO4 cell does it no harm.
One each of these male/female polarized disconnects, one for the panel and one for the batteries. This allows the Lionel base to be completly set aside without removing the panel.
Electrical tape prevents cutting the wires that come off the solar panel and wrapped around one AA to D adapter marks it as a dummy. I also tape the Lionel battery C-clip so that it doesn't bite and short regular alkaline cells when they are used.