Happens after a while and use: the clips from the motor housing break off.
Usually it is not really critical as long you have just the Lego® standard trains, but if you do your own train MOCs and do some details on the bogies, these small clips can be very helpful, because the motor housing has not much to offer regarding studs or other ways to fix something on it.
Below, I will show you a way to repair the clips on the motor housing. The good news is, it is not necessary to open the housing for the repair. The lesser good news is, the repair is done by Lego®-foreign parts, in case this matters for you.
Here the instruction how to do the repair step by step (free of charge):
Disclaimer: I am not responsible for any damages or injury on parts and humans happen while following my instructions. Everything you do is your responsibility and under your personal risk.