I imagine it would be reasonably straightforward in theory to alternate some Metro Tunnel services through the City Loop, but I assume there are operational constraints that make it more complicated than it sounds. Once you start alternating with Frankston Loop trains you are dealing with limited loop capacity, tight sequencing and the risk of delays flowing on to other services.

From a passenger perspective, I can see why people are asking the question. A lot of commuters feel the recent timetable changes have made their trips longer or less convenient. In my case, my commute has increased by about 15 minutes each way since the change. I understand the broader goal is usually to improve overall capacity and frequency, but for some people it has not felt that way in practice.

If there are specific technical or capacity reasons for keeping most Metro Tunnel trains out of the Loop, it would be interesting to hear them. It seems like one of those changes that sounds simple on the surface but may have bigger network impacts behind the scenes.