How I can lubricate the 4.5v train motor on my #7722 train set? It squeals.
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.
I used this technique (if the video disappears: a tiny amount of lubricant directly on the rotating bit of the engine that the axle with gear sticks out of, so between the metal gear that's fixed to the axle and the engine housing) with exactly the same effect as in the video: the high pitched squeaking noise disappears completely, immediately. I probably used the wrong oil, I think it's best to also do this with something that won't damage the plastic, but on the other hand I doubt very much will make it's way out of the engine and the metal parts surrounding it onto the plastic gears or housing.
Note that you can test first if the engine is the noisy bit by holding the power to the contacts at the end of the engine after getting it out like in the picture in Tonny's answer.
Too bad I don't know how to open my 6216m engines to do the same thing there.