I think it's a very bad idea:
You are taking someone with a safety critical role, in a highly skilled job, and asking them to provide a purely customer service function, with product lines and methods of service with which they are not familiar, where this function could be achieved by an employee with much more limited skills.
You are likely to get poor customer service (sorry, you can have the soup in 5 minutes when I've dealt with this alarm/station stop/...) from an employee who doesn't want to do the job (I spent months training to serve instant coffee??), and would therefore be disenchanted and harder for the company to keep good employees.
The real solution is to look to continental experience (Norway, France...), where a coffee / snack vending machine is provided on the train, with a full catering provision when journey length / clientelle / time of day are appropriate, thereby shifting your staffing costs to the times of day where they are most likely to make a profit, while continuing to provide a limited catering service 24/7 where appropriate.