Based on my experience playing Railway Empire, passenger cars with people and their luggage is significantly lighter than a freight car of the same size loaded with cargo. This means it takes a lot more energy to get the freight moving at a higher speed, and maybe more importantly a lot more to stop (I think it takes 3-4 football fields for a loaded freight train to stop from 30mph). So just having passengers allows the train to travel at much higher speeds. Speed is something more valuable for passengers because they want to get where they are going sooner. Freight is more about total throughput volume so it may be better to have one heavier train carrying twice much at a slower speed than two trains have the size each moving faster. So while you could have a mixed train it's not going to be as fast as the passengers want due to the heavy freight cars slowing the train down and won't carry as large a volume as the freight customers want because some cars are being taken up by passengers.

Freight and passenger trains optimize for very different things, and those things are largely incompatible.

Passenger trains want speed, quick turnaround on the vehicles, frequent stops etc.

Freight wants efficient transport (= lower speed), few stops, turnaround time is less important.