Both practices are valid. To be precise: you subjectively feel that meditation on the public transport to be sharp, more concentrated, and efficient, but you suspect that this subjective reaction is questionable.
Let's draw a comparison another way. Suppose you compare 2 sits on the public transport to one another. Or, two sits at an Advanced Meditators retreat to one another. We'll say we compare 2 sits which are in identical settings, of equal duration; sit(1) you diligently follow the instructions but the experience is uncomfortable, bland, and you don't see any benefits that day. Sit(2) has pleasure which is spiritual in nature, or clears out the mind, or has only positive effects all day long. Yet both were identical practices?
It is possible that the bland meditation without noticeable benefit may be just as, or more valuable, to your practice, then the pleasurable one. That's because it's a process within a system, and you can't see all the benefits. Of course we WANT the later experience, but it does not mean it is actually more productive.
It could be that your sitting at home in quiet and seclusion is dealing more deeply in some areas you might prefer not to go in? Or could it be the public transport is easier because you are more distracted?
In my own experience, I really treasure the less pleasurable sits because they are turbulent due to the things bothering me, and I feel that stuff needs to be front and center. It might be that your sits at home are of more actual benefit, but it doesn't look that way now.