These trips started as long weekends for a group of friends. The first trip by car was to the first world war battle fields of Ypres and the Somme. It was also an opportunity to travel through the channel tunnel just after it opened. The rail trips came later. The trip budget increased but mainly the availability of information on the internet along with online booking of rail and hotels made these possible. The first trip was Derby to London to Paris to Geneva to visit CERN.
The Trip Planning Process
Train travel is relatively expensive so travelling on discounted tickets is an important aspect of planning. Different tickets go on sale at different times, Eurostar are first, then TGV routes and finally intercity routes in the UK.
Eurail
The last trips we made using Eurail Passes. These may not be cheaper than the advance tickets price but can make the overall trip cheaper. For example the pass cost and reservation fee for the Eurostar is fixed with a pass but the ticket price varies with the popularity of the train. The Eurail site let's you book more than 5 people on a train where the Deutsche Bahn site for example has a maximum of 5 persons on a booking and you have to make 2 bookings if 6 or more are travelling.
One return trip in your home country is valid with a Global Pass so if you need to catch a peak time train to St Pancras you will not have to pay the high ticket price. Eurail passes often have a 10% reduction in March, you can buy the pass and travel any time in the following 12 months.
Routes and Timetables
The first place I look when I start to plan a trip is https://www.seat61.com/ that will give me the routes and good background information. Then I look for timings and stop overs using https://raileurope.co.uk/en and https://www.bahn.com/en/view/index.shtml both these sites cover all of Europe. https://www.thetrainline.com/ is also now good for international planning (I never used The Trainline as it originally charged a fee for tickets) .
The detailed planning I always do from the train company web site. Where possible I always but tickets from the train company site. My experience it never costs anymore and there are normally associated benefits. Cancellation, moving seat reservations, rescheduling a journey claiming compensation I have found relatively easy for travel booked directly with a train company.
Train company web sites can be hard to navigate, sometimes information is not available on the English language part of the site and you have to go to the local language and flick between screens to understand what it all means. Town and station names can vary so make sure you use the local not the English spelling. Google it first and find the station on a map. I think all this part of the fun but I appreciate it is not everyone's cup of tea.
The local site will charge you in local currency so you ideally need a card that doesn't charge a foreign transaction fee I use a Santander Zero or a Starling card which give good rates of exchange and no fee. And check that the site that is taking your money is legitimate.