This is the charger i have
Titanium Smart Fast 16 Bay Ni-MH AA/AAA Battery Charger AC 100-240V + DC Adapters MD-1600L

Eneloop spacer pack C to AA

Never thought about the weight of the C battery affects the locomotives.
I guess i have not really noticed. maybe it just goes faster a little with less weight 😃
Right now I only have 64 AA and 16 AAA batteries.