不劈腿的美人鱼不出轨的小火车
[bù pī tuĭ de mĕi rén yú bù chū guĭ de xiăo huŏ chē]
It seems to play with some imagery, 'the mermaid never cheated on the prince' meets a little 'little train staying on its own path', both suggest loyalty and fidelity to certain principles, people, or goals set up in one's own mind, emphasizing steadfastness over time.