What is the margin between trains?
How long does it take a passenger train to clear the tunnel?
What is the maximum speed through the tunnel for a passenger train?

There is normally up to 4 shuttles & 4 freight shuttles per hour. Add the Eurostars & the odd freight & u can see that this adds up to about one every 5 minutes. They try to move as much freight as possible at night of course, when the passenger traffic is low. Transit time is given as 35 minutes for a passenger shuttle, including loading/unloading.
With trains so close together, everyone has to move at the speed of the slowest, so the practical running speeds r lower than the theoretical - Eurostars do not go through at 186mph.