Big no-go for me on the batteries as a required power source, for many of the reasons already covered.
I'm mostly a command runner, though I do have conventional trains I run as well.
For Lionel TMCC/Legacy, I've always been fine with the optional battery, as for command operation, it doesn't affect much. It doesn't hinder operation in any meaningful way (to me) if the battery is not there. The only modern Lionel engine I own that has ever had a 9v battery installed was the Century Club GG1, and only because Lionel decided to install one at the factory for some reason. As soon as I learned that, it was removed.
For MTH, I don't recall them highly advertising the fact that there was a battery inside required for successful operation in the early days of PS1. I'm sure it was always covered in the manuals, but it was probably a while before those first batteries came to start to no longer be chargeable and needed replacement. Limited catalogs back then certainly did not have a rechargeable 9v battery shown in the accessories section of the catalog. And it was later in PS2 when such a concept as the external charging jack came about so you could charge without being on the rails or opening up the engine to remove the battery and charge it with a separate charger.
I accepted the battery as a necessary evil once I knew about it for the MTH trains, as I felt the value of play with DCS (or even PS1 - I still love some of the specific sound effect sequences built into things like the Shay) was enough of a benefit that tolerating the battery (knowing it would eventually need replacement or a BCR) was worth it. I don't feel it's a coincidence that MTH (finally!) migrated away from the battery with PS3. (nor is it a coincidence that the BCR units that preceded PS3 became so popular as a replacement for the rechargeable batteries for MTH trains). A large percentage of the public who bought these trains saw the battery as a big negative.
Without digging too deep into the technical differences in amp-hours, charge times, etc, it seems to be pretty clear that batteries can be readily implemented for anything that is just electronics with a bunch of relatively low power draw processors and such processing all the 0s and 1s. (cell phones, tablets, etc - though obviously there are also analog RF and display circuits involved, but not any/many motors). Anything producing enough power to provide mechanical motion (at least where there is significant weight and therefore some torque required) seems to be either limited in run time per charging session (RC car examples provided) or extremely expensive to produce the technology (Tesla example). I don't want my trains to increase in cost proportionally the way the cost of a Tesla compares vs. a Chevy or a Ford 
.
Seeing the reference to a battery the size of an SD memory card seems extremely ambitious to have enough power to move a train. I'm just thinking out loud here about the number mentioned for the Tesla battery. It looks like a Tesla weighs between 4000 and 6000 pounds (depending on model) with the ~1000 pound battery. Scale that percentage to an O Gauge engine weight. Not likely to ever fit in the size of an SD card.
I know technology evolves, but I've seen too many "gotchas" with various battery tech over the years to get excited for batteries anywhere they are not necessitated (necessary for things like cell phones, cameras or portable radios, of course). Years ago it was the Ni-Cad "memory" effect (probably made worse with "quick chargers" (quick back then was 5 hours instead of 15). Then more recently (maybe even current) for NiMH batteries, there comes a time when they just no longer like to be charged. I use some digital cameras that (thankfully) use standard AA size batteries, but I've had many AA batteries that just eventually give out and no longer play nice in the battery charger. Charger decides that there is something it doesn't like about the battery and it won't even start the charge. Not the end of the world since it's standard AAs that are readily replaceable, but still not optimal. (and I'm pretty sure there is no way I've exceeded the advertised number of recharging cycles).
So if O Gauge trains ever come about that use a primary power source of a battery instead of the rails, I will not be buying, no matter how cool the train itself may be.
-Dave
