This has taken an interesting turn. I have with me an EarthX LiFePO4 battery, approved by the FAA for aircraft use. Expensive as all get-out, and I know nothing about it (yet). It is so light you would think it was empty. 16 amp hours. A normal 12 amp hour battery will start my aircraft 70 times between charging.
I bet that a tender-size battery of the same type would be the next step. But at $600 a pop, not yet ripe for model trains.