My train was dispatched yesterday.
I looked up the power functions myself, the Canadian listing for the train gives a bit more detail. These are the parts you need:
To motorize the engine, purchase items 8879, 8884, 88000, and 88002. Instructions can be found in the back of your building instructions.
https://shop.lego.com/en-CA/Winter-Holiday-Train-10254I skipped the remote control parts and just ordered the battery and motor (£11.99 each if memory serves). They have a long lead time though so I won't be getting those for a month or so.
Looking at the picture for the battery pack, it looks like it has a switch on for manual forwards/reverse operation... Hopefully you can access that switch when it's in the train