Two issues there:
1) The driver is unlikely to have the knowledge to be able to answer "customer service" type queries. Being an ambassador of the company doesn't really help with answering ticketing questions, for example.
2) More importantly, in many cases the driver doesn't have time to take queries, especially if changing ends on a tight turnround, or whilst trying to work their way through a train defect. One person's query doesn't take priority over the train departing on time.
1) No of course not, but they could signpost people to the right place "Oh just ask my colleague over there / at the ticket office". And they may well be able to answer a question to "Is this the train to X". Drivers are staff like everyone else and should be courteous with any enquiry not shrug it off as some feel is OK because they are 'above customer service'.
2) Again that can be handled with an firm but fair "apologies, I need to prepare my train for service, could you please ask one of my colleagues over there / at the ticket office / on the platform"
A little bit of customer service doesn't hurt and indeed drivers go through the same customer service induction processes as everyone else.
