It is hardly eavesdropping because the conversations are loud and uncensored by self-consciousness, etiquette, or even the vaguest flickering of political correctness.
Surprisingly, most of their conversations are about their college programmes and their lecturers. They are deeply involved in the minutiae of analysing resources, the character of their tutors, equality and ethical issues, lesson structure, and lecturers' ability to motivate.
In fact, they are so good at evaluations that the whole of the Ofsted and the Adult Learning Inspectorate could easily be made redundant by the simple device of adding a soundtrack to the security videos on trains throughout the country and posting an appropriate selection to the Department for education and Skills.
How did students get so good at it? It takes months to ensure trainee teachers do decent lesson evaluations and proper peer observations. It takes years to rise to the dizzy heights of being a teacher trainer and longer to become an inspector.
Students just seem to have the knack of making evaluations without effort.
They are good at it, even when they are not travelling.
An A-level modern languages student once came to see me about changing to philosophy. His account of his French teacher's inability to provide well-structured, motivational lessons was detailed and precise. Weaknesses were balanced with recognition of strengths, but the overall assessment was pretty damning. How did he do it?
Mature students are more frighteningly articulate. Even before they have studied a subject or learned a skill, they can critically evaluate the overall course, teaching methods, speed of delivery and assessment models.
How do they do it? Even lecturers with years of teaching experience find such critical thinking hard but now it seems everyone of a certain age can do it.
Of course they cannot. Student evaluations are entirely worthless. This is not an empirical point based on any research that looks at what they say.
Such a project would be pointless. I'm making the logical point that to be able to evaluate a course, or even a lesson, students would need knowledge and skills at least equal, if not superior, to their lecturers.
Not only do they not have the ability to evaluate, they cannot have it. It is just a false and flattering pretence to say that they have anything to say, or to ask them questions of the easily quantifiable kind with which we are all familiar: "The academic support on this course was very good, quite good, neither good nor bad, fairly poor, very poor, don't know".
Students should all tick "don't know" every time. That is the right answer.
They don't know. Only part of the excuse for this indulgence is that students are increasingly seen, or see themselves, as "customers".
Students can never be customers because customers know what they want, whether Rolos or a Rolex, but education is different . You cannot know what you are going to get out of it in advance, however much you pay.
Most colleges will have already gone through the annual panic when courses and methods are modified to take account of student evaluations. This is a process that undermines lecturers' authority and leaves them abject in the face of student whims. Often changes are made one year and then reversed the next.
Some lecturers even offer students the chance to undermine their authority by encouraging a form of therapeutic self-expression, in which they ask them what they think of their lesson without comment. The problem is we let students opine and do not challenge their competence to criticise.
It is time to get a grip and say "No" to every suggestion in a student evaluation. Of course, this will earn anyone brave enough to do it a low score on the student satisfaction index. But years later, when students can really assess the impact of their education and training, they may at least have a guilty conscience.
I remember one teacher, Mr Bowness, who terrified us all. If there had been pupil evaluations then, we would have requested his removal. Looking back he was the teacher that taught us most, though we would never thank him for it. Fortunately, no one asked us, and he presumably went on to knock knowledge into generations of recalcitrant pupils.
Students can say how satisfied they were about the quality of sandwiches in the canteen or the range of products in the college shop, but what is the point of asking them about their education or training? They just do not know.
There is one exception. Listening to two trainee car mechanics on the train one morning, one said: "Last year was all right but next year will be bad."
His mate pressed him and he explained: "It's that Mr Brown. His lessons are well 'ard."
That is what we should aspire to if we really respect our students, no opportunities for opining, just more lessons that are "well 'ard".
Dennis Hayes is head of the Centre for Professional Learning at Canterbury Christ Church university