Always speak to staff. They're obliged to offer an alternative without extra costs and if you get stranded, they'll have to organise accommodation or alternative transport e.g. by taxi. The best is not to wait with that until you strand but e.g. speak to staff on the train and ask what to do or where you should go to get help.

I guess in my case, part of my mistake was trying to fix things myself when I learned about the ICE cancellation. (I was still in my hotel room in Brussels, and I saw that the Thalys was my only option for getting to Chamonix that day, so I bought the supplement guessing it would disappear quickly!)

Maybe I should have gone to Midi and found the DB rep there, but that would risk no space on the Thalys and a definite overnight delay.

Would somebody in Brussels really be able to offer a solution to a journey two countries away?

My guess was that any DB person in Brussels wouldn’t be able to put me on the Thalys. They’d just say ‘take the 1025 DB departure, ask in Köln for your connection’.

Indeed, never try to fix things yourself if that involves spending money. Getting that back may prove difficult if you didn't first ask for help.

In Brussels they would have been obliged to offer an alternative for ICE13 free of charge at the least and also advice on what to do if their alternative would mean you'd be stranded somewhere.

Have Brussel DB staff? Or does SNCB/NMBS help in this case?

There was a DB guy standing by a table outside the ticket office. He had a long queue of people wanting to talk to him (about the cancellation I guess!).

Personally I prefer to rely on my own common sense. It is very hard to get practical help as the staff at stations often don’t have the authority to get hotels. You’ll be told to pay and claim your money back. This is my philosophy the travel is very cheap and a lot of fun. If it goes wrong I have a credit card and might or might not get my money back. I like to be in control and apps give so much information. I’m just back from travelling from Venezia to London point to point tickets. Just about everything went wrong in Austria and Germany and ÖBB offered to refund a hotel night in Munich. DB staff on trains were very helpful after a series of cancellations and we did manage to catch our Eurostar. The only on time train was the one in Britain! Long distance train travel can and often does go wrong just be prepared. 

You will find it differs. In Germany for example, staff can and will offer hotel rooms. In the Netherlands, taxis are normally offered for any domestic destination and hotels for stranded international travellers. NS does not reimburse you when you did not contact them first (and their customer service is open 24/7).

Of course, whenever you ask for assistance, it's good to know what you would like to happen so you can possibly get that. If the staff can't or won't help you (some just can't be bothered), then you can still make your own arrangements.

I thought I would update this thread.

I made a claim to DB for the €75 of expenses, and they quickly passed it to Eurail. Eurail said they weren’t responsible, and after some emails back and forth, told me I had to ask DB again. I did, sending them a letter by post (the only way I think to contact them?!) and they refused again.

This time, they say they’ve passed my claim onto Thalys and SBB. This doesn’t make any sense, because Thalys didn’t do anything wrong! SBB perhaps makes sense, since they were the operator at the point I needed assistance, but it was DB which caused the delay.

Madness!

Eurail should pay the extra costs, since they are the "unavoidable costs incurred in order to reach the original destination”, as specified in the delay compensation policy (point 3).