The reason people look to battery trains is to avoid the infrastructure costs of the overhead lines / third rails, substations etc. to get electricity from trac

The reason people look to battery trains is to avoid the infrastructure costs of the overhead lines / third rails, substations etc. to get electricity from trackside to the trains. The costs can be difficult to justify for smaller branch lines. A train that can use overhead lines on the main lines, then switch to battery for a branch is (hopefully) greener / more efficient than say running a diesel under the wires all the way[1]. It is a bit of a case of shuffling costs from infrastructure to rolling stocks, trains with batteries would still cost more than a pure overhead line electric, but I guess it comes from a different budget... I'm not sure of any real reason why you'd use a battery off-track instead of a connection to the electricity grid except as some sort of backup for a power failure? [1] Bi-mode diesel / electrics also exist of course, which solve the problem in a slightly different way (insert NOX / CO2 v Lithium mining debate here).