When the CDV-700 was made over 50 years ago they were intended to be powered by carbon-zinc D-cells. Carbon-zinc cells are no longer made, so alkaline D-cells are usually used in their place.
This switch has 2 unintended consequences. The first is added weight as modern alkaline cells are much heavier than carbon zinc and the second is the risk of an internal short in the battery holder. Victoreen 6/a/b models use 4 cells, Lionels 6b's use 2 and Anton models use 5 cells.
The battery holders on Vics and Lionels have a wire crossing underneath to connect adjacent cells in series. The cell pairs in the holders face the same direction. Alkaline cells have steel cases with varnish labels. The metal c-clip that retains adjacent cell pairs will cut through the varnish and short one cell to the wire underneath. Carbon-zinc cells were wrapped in paperboard with a foil label that did not touch the anode, so there was no risk of shorting carbon-zinc cell this way.
Step 1: The Hack
Recently lithium iron phosphate AA cells have become available. LiFePO4 cells have double the voltage of alkaline cells (3.2v) and will not burn if shorted like lithium cobalt (3.7v) cells will.
2 lipo cells, 2 dummy cells and a charger for $20
Plastic AA to D cells adapters are available on eBay
Less than $1 each shipped. The plastic shell won't short and it weighs less than the original.
I have nickel strip and a battery tab welder, so instead of using dummy cells in the d-adapter I tab welded a nickel strip to make a d-sized dummy cell. I wrap my dummy d-cells with electrical tape to seal them and mark them as dummies.
In the Lionel, one lipo cell in a d-cell adapter goes in one slot and the other gets a dummy d-cell made with a plastic case and a dummy aa or by soldering a wire/nickel strip across the contacts.
ON THE VICTOREEN MODELS there must be one real cell and one dummy in each battery holder or damage will result.
The run time with this hack is comparable to the original carbon-zinc cell, with no risk of leaking alkaline chemistry.
The cost of a rechargable lipo cell is less than a single pair of disposable alkaline d-cells.