Please be aware that a couple years ago, Learning Curve changed the design of their battery trains. The new trains had the ability to free-wheel when turned off (previously this was not the case so if the engines were off they could not be used like normal wooden trains).
This change in design improved the useability of trains allowing normal push play as well as unassisted battery powered operation. It did however mean that the cogs (instead of being engaged at all times which prevented moving when switched off with older trains) have to engage and disengage when the train is switched on and off. The cogs can occasionally engage incorrectly and stick in such a way as to prevent the train froom running.
How to fix the problem: If your battery train will not go when you switch it on, the problem could well be caused by the cogs engaging incorrectly (of course the battery could be flat and require changing). If this is the cause, the problem will be resolved by turning on the battery train and gently pushing it forwards. The engine should spring into life and normal play can be resumed.