I agree, exactly what Chrisjo and Don9Doug say above. If these were all bought from the same fellow who bought them for his child they show the exact signs of a child holding the train still while running which will do something with any number of gears either spinning, splitting or cracking. The tell tale sign of an engine slipping on an incline especially slipping while pulling nothing is a cracked gear. What I am really curious about is the first video where the engine doesn't make it 'Down' the slope and looks like there is no slippage but the motor has cut out, don't recall this before. Is this the case?
BTW...welcome to BPT's ☺️
BTW...welcome to BPT's ☺️