How to lubricate the 4.5v train motor for set 7722?
Undo the 2 screws at the top and then take of the top cover.
You can now take the actual motor out  of the casing. It just sits loose in there.
Then you can undo the 2 screws in the bottom to free the metal block with the plastic gears.
It is most likely those gears that cause the squealing because of dust/dirt that got into them.  

I would recommend to clean them first using a soft brush. (Paint-brush works well). You can stick an axle through the bushings and turn the gears manually or rotate the big gear shown at the top with your finger. Just let the gears run against the tip of the brush.
Don't stick the brush in too deep! You don't want the hairs of the brush to get caught in the gears!  

Then lubricate with a little bit (a few drops is enough) of plastic safe oil. the movement of the gears will spread the oil around eventually when the whole thing has been put back together and is running again. (This might take some minutes of running though. Don't expect the noise to go down after 5 seconds!)  

Don't use WD-40. It will degrade the plastic !!! 

You should NOT lubricate the actual motor itself. Some of the oil you put on the gears will eventually make it into the electric motor and that should be enough.