Would bakers be willing to branch out to non bakery items though? I'm talking alcohol, crisps & chocolate, Haribo, Noodles, tapas snack boxes. These are all things that the customer expects of on train catering, I can imagine a bakery wanting to sell bacon rolls and tea and that be it, but having that service could actually increase complaints from people like vegans and gluten free people, whereas specialist catering companies i.e. FISCO are very good at meeting the needs of all passengers and at all times of day. Perhaps if Greggs got a liquor licence it could work, but the average cob shop chain wouldn't have the supplier connections to expand their range.
There was a local baker who branched out into selling stuff in industrial estates in Northampton in touring vans. They managed to sell hot and cold butties, and a range of other stuff.