It really depends on the railway line. A lot of Europe is running enough passenger trains that there literally isn't enough space to add more cargo trains into the mix.

In my country the minimum train-train spacing is five minutes, because any less and a train won't be able to stop if the preceding one has an accident. A substantial part of the network is already being limited by this, and we are seeing more and more quad-track sections.

Your "carrier train" already exists, to an extent. It is called "intermodal shipping containers".