How can I complain about the quality of service I'm receiving from a train company?

You should first complain to the train operator concerned and seek a reply.

If you are dissatisfied with the response you receive, you should then contact Passenger Focus and ask them to investigate the matter.

The Department for Transport is responsible for ensuring that train companies run services to the required standard within their franchise agreement and comply with certain consumer protection conditions in their licences. DfT can take action against operators who fail to provide an adequate level of service.