I am intending to do something sort of like this in the 2nd release of our Locomotive Controllers. I believe it to be full of potential problems as mentioned about shorting out the track power. Another is that Lithium batteries are not charged by hooking up to any voltage.
You could potentially use track power to power the motors when the track power is greater than the batteries, but there would need to be blocking diodes to prevent putting charge back into the battery.
I think there is a possibility of making a charging siding or spur that a locomotive could move into, then an onboard charger could control the charge into the battery. This is not the same as charging on the regular track using standard track power.
