Just to clarify, it's double track from Ipswich East Suffolk Junction (and the new Europa Junction) to Westerfield, then single to Derby Road (which has a passing loop), then single to Felixstowe passenger station. The single line to North Terminal splits off at Trimley, the one to South Terminal just before the passenger station. There is no passing loop at Trimley. I *think* there is a set of intermediate signals between Derby Road and Trimley to allow trains in the same direction to follow each other through that section.
The port website has a train schedule -
https://www.portoffelixstowe.co.uk/rail/rail-schedule - but of course as D6975 says, not all trains actually run on any one day.
As to how it works - in practice the people 'on the ground' (signallers/drivers/conductors/dispatchers etc) make it work, but there is very little slack in the system to cope with things going wrong. When the passenger trains are running, quite often a freight will closely follow a passenger train - it has time to get clear of the 'mainline' while the passenger train is at Ispwich/Felixstowe. Sometimes it's possible to squeeze two freights between passenger workings (especially if they are to/from different terminals).