Europe by train

I don't know that anything needs to be bought in advance, but a few of these you can save significant money if you do buy nonrefundable train specific tickets.

Worms to Boppard is a Rheinland-Pfalz that can be done with regional trains. Fast trains will not save time or money.
Amsterdem to Delft, Delft to Antwerp, Antwerp to Bruges are IC trains which don't need reservations and have little or no savings buying ahead of time.

Cologne to Amsterdam, Bruges to Paris, and Paris to Basel should be bought in advance, and the sooner you do, the better your price.

Schliengen to Worms, Boppard to Cologne may or may not save much money buying in advance. A lot depends on how many and the ages of the passengers.

Oh!! Forgot to add that info. There are 4 if us. Ages 55, 55, 25, 23!

And thanks for the info!! Very helpful!!

In Germany you can have considerable profit by using their so-called Ländertickets (Regional tickets) when travelling as a group. Worms to Boppard can be as cheap as EUR 39 for you all together. The conditions are simple: only valid on regional trains (not IC or ICE) and after 9 AM. Just make clear at the ticket window that you're travelling as a group of 4.

and the 9am restriction is only from Monday to Friday.

For instance, Schliengen to Worms can be done in 3 hours with 2 train changes for 120 EUR if you buy Sparpreis tickets now that lock you into a specific train on a specific day. If you change your mind, you can get a refund by paying 19 EUR fee up until the day before your travel date. Then you have to pay the going rate for new tickets, which is 234 EUR.

On the other hand, if you can accept taking 4 hours to do this journey, you can travel on regional trains with a Quer durchs Land ticket for 68 EUR for 4 people. Just leave after 9 am. If your travel date is on a weekend or a holiday, you can do it for 62 EUR using a Schoenes Wochenende Ticket and you can travel anytime. You may have to take an extra train change as well, but I see one route leaving Schliengen at 9:16 am, changing trains in Offenburg and Karlsruhe and arriving Worms at 1:19 pm. With these tickets, you buy day of travel and are not locked into any train routing except all must be regional trains. No ICE, IC, or EC trains.

This page has links explaining these regional train deals.
https://www.bahn.com/en/view/offers/regional/index.shtml?dbkanal_007=L04_S02_D002_KIN0060_ST-REGIONALE-ANGEBOTE_LZ01

When you use the www.bahn.com travel planner, click on "local transport only" to see these train routings.

Sam, again, thank you!! This is exactly the information I was looking for. With so many ticket options, I wasn’t sure what was best for what leg. This helps immensely.

Oh, other questions (info. I forgot to include, I apologize). Found out my son and DIL are considered EU residents, (having lived in Germany over 6m. they’ve been there almost a year working at a school. We’d ask them these questions, but they do most travel through, and arranged by, the school). Does this change anything for their tickets??

my son and DIL are considered EU residents

They will not be able to buy or use a Eurail pass. The could use an Interrail pass, but passes are rarely cost effective anyway.

In Germany, Länder-Tickets and SparPreis Tickets are available to anyone.

If they went to trouble to buy Bahn Cards, either the 25 or 50 percent discount ones, then they would get the applied discounts. If they just went along on school provided travel, I doubt if they got them, but ask them, by all means.

Bruges > Paris
Paris > Basel

These both need to be bought in advance, and the sooner you buy the cheaper. The Thalys from Brussels to Paris has the same rule as the TGV to Basel, the tickets are specific to the train and have a reserved seat number on them.

Chris, thank you!!!

As well as Cologne>Amsterdam
I see 4 tickets for 80 EUR in July if bought now. Walk-up price is 274 EUR.