At EMRC we use two battery chargers ( with modifications) to power our trains. We use a split potential power system that means -12V DC 0V + 12V DC. In effect you are using one transformer in one direction, and the other for the reverse direction.
The battery chargers are 4 amp units. To date we have not had a power supply problem with too many locos on. At times we have had 4-6 separate locos moving etc.
See the power supply picture for a example of a split power supply. See the second picture for an overall diagram of the layout's wiring.