With regard to battery type, when I built my first battery powered loco in 2014, I used a 2000mah NiMH battery on a trailing car. It worked fine, is not as fussy as LiPo, but would not easily fit in a GP-9 or RS-3 shell. I switched to LiPo (1300mah) so I could get the battery on-board the locos. I provided small jacks conveniently located for recharging. I am careful to monitor the recharging current when recharging. So far, everything works fine.
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