不会劈腿的是美人鱼不会出轨的是小火车
[bù huì pī tuĭ de shì mĕi rén yú bù huì chū guĭ de shì xiăo huŏ chē]
It suggests loyalty - like the mermaid that doesn’t split its legs into a human form (which would symbolically mean cheating), and small train cars staying on the track faithfully.