Just an idea that might help Interrail users.
We live about 400 km from Southampton and have cruised from there a few times.
We are considering using an Interrail Global pass in conjunction with our next cruise and the numbers seem to compare with the alternatives of driving and parking or flying from local airport.
To travel by peak time trains would be extortionately expensive or inflexible, but allows unlimited luggage and avoids a long drive.
A four day pass would be just over 200 pounds and no UK res costs.
2 UK days and then 2 days of tickets to use in ports e.g. Lisbon to Sintra or Le Havre to Paris. Compared to cruise excursions this saves another couple of hundred pounds.