We'll be in the Netherlands, Belgium and Germany on our summer trip. Should I consider reserving the train trips between Leiden, Netherlands and Bruges, Belgium, Bruges, Belgium and Boppard, Germany. Some of these may involve IC, Thalys, or ICE trains. If reservations are advised, which web site is best for each journey? And will there be any issues with US-issued CC online? Also, from what I've read, I won't need to reserve for the in country day trips (i.e. Leiden to Delft, Amsterdam, etc). Please confirm.

You do not need to book for in country trips. Ticket machines in the Netherlands now accept credit cards though there is a surcharge I believe.

Seat61 is an excellent resource for train travel.

For international train tickets look at Belgian trains NS International or DB

Yes no reservations needed on those trains though you can make optional reservations on trains in Germany but Belgium and Dutch domestic trains do not even accept reservations except for Thalys trains which you need not take. www.seat61.com yes is the best informative train site - others I like for general info on trains www.ricksteves.com and www.budgeteuropetravel.com.

Thalys: reservation is mandatory. Not a question of advised or not.
IC in Netherlands and Belgium: I have not bothered with reservations within Netherlands or Belgium. I don't know if it is even possible. These trains run so frequently.

IC/ICE in Germany: This depends. On trains leaving a major city heading to another major city during the morning/evening rush hours, I found it was often quite a pain to find seats not reserved for our segement for two -- meaning I had to go from one car to another to find availabe seats. When I travel during mid-day, I don't bother with reservations. When I travel in a party of four on an ICE, and we want a table, I reserve. On each car, there are only a limited number of four sitting face to face with a table in between.

I have had mixed results with US credit cards. For online purchases either on a web site or using an App on my phone, my BofA Visa card seems to be acceptabed more than my Citi Visa card. Citi Visa sometime clams up when I tried to buy many train tickets/reservations in quick sequence. For vending machine purchase in Netherlands, I have only used my creidt union MC as it is the only card I have that can do a PIN based purchase transaction. Yes, the Netherlands train vending machines tack on a surchage if you buy tickets at a station with a cc.

Booking IC trains in the Netherlands/Belgium in advance will now give you a lower fare. You book a ticket for a day and can get any train. No reserved seats. If it's rush hour you may have to stand.
So if you know you are traveling on a certain day, book the tickets.

For example, if I want to take the Intercity Antwerp-Amsterdam tomorrow, I'll pay 37,40 one-way.
If I now book the same train later this month, the ticket costs 20 euro. First class is 28 euro; worth it for only 8 euro extra.
The weird thing is that if I want to go Antwerp-Amsterdam tomorrow, Thalys is only 29 euro one-way (45 first class).
This is on www.nsinternational.nl

I don't think there's an advantage in buying in advance for local trains, but definitely for IC between the Netherlands and Belgium.

Regarding the journey from Brugge to Boppard:
When you look for this connection on bahn.com, you will most likely be taking the route from

Brugge to Brussels: Belgian IC trains (never used reservation on those, not sure if you could)

Brussels to Cologne: ICE or Thalys train
Reservations are not required but recommended -- On bahn.com you'll see an icon next to the connection for ICE trains- predicting the availablity of seats; also check fares in 1st class - while saver fares 2nd often sell out quickly, those in 1st class often don't - and often the difference between 1st and 2nd comes down to a few euros.

Cologne to Boppard: Regional trains, no reservations possible.

I'd book this whole journey in advance because the ICE and Thalys train fares can go up and up.. so you really save a LOT when you buy in advance.
As German and Belgian Rail both offer cross-border ticket sales you can check both b-europe.com as well as bahn.com as see which one offers the best fares. Regardless which company you choose, your ticket will be valid for the whole journey from Brugge to Boppard.

For trains within Belgium, there is no advantage to booking in advance. Book online and print your ticket; it's valid for any train on the route that you booked on that day.

do you have to validate your own open ticket before boarding the train?

Depends on where you are. If you have a Dutch ticket, it's really a one time, disposable chipcard, and you have to validate to open the turnstiles to the platform. That's not the system in Belgium and Germany however, where your ticket is checked on the train.

I wanted to circle back and thank the travelers who responded, especially with their personal insights and also to share what I did in case it helps anyone in the future. The website www.seat61.com is also good of course, but a little overwhelming.

Since I had fixed travel days moving from country to country, I did book first class tickets in advance. I hope the prices are more reasonable; I ended up avoiding the THALYS options because they cost twice as much. Except for the ICE portion (Brussels to Koln) all the trains had unassigned seating, just like some of you mentioned, so I hope first class will not be crowded as we will have a decent-sized suitcase each. There are a couple of tight connections (7 minutes(!) in Rotterdam), but we could always catch the next train I suppose. And we may catch an earlier train to Brussels from Bruges so as not to miss the ICE train from Brussels to Koln. That's really the only potential problem though there are 16 minutes to get from track 1 to track 5. In theory, we should be fine, but if the train is even 10 minutes late, we'll be stressed.

We plan to do day trips from our base cities by train, so we'll have had some practice with the trains by then. Rather than validation stamps like I remember in Italy, there are images to scan on some of the tickets to open the gates. Also, in two cases, one ticket clearly states it is for 2 people and in the third case, there are individual tickets.

I also booked a regional train --not a high speed one-- direct to the Frankfurt Airport, where we'll be departing from. Even though the regional train may not be as posh or fast, there are no transfers, and it's only 90 minutes or so.

Finally, while I called my CC company to advise them to expect the charges, I did get one fraud alert text that I had to respond to so the charge would proceed.

Thanks again!

Nice to now just show up for the train! Sweet! 1st class can be full on German train these days - and 2nd class often quite full - you can make optional seat reservations and I'd advise you to do so if you did not - can do once you arrive in a city for outward trains - just a few euros a seat - regional trains may have no seat reservations and starting from Cologne it should have lots of seats though be sure to board ASAP.