The operator where you get stranded is responsible for assistance, be it overnight accommodation, alternative transport, meals etc.

Always speak to any staff available. If there's no staff on the train, then look if you can contact customer support via phone or any other means. In the Netherlands for example, they can be contacted 24/7 by phone or by pressing on the info button of the information column, which is present at each station. In Germany, you can often also contact a regional back office; phone number can be found at the station.

If you can't find any help or if staff refuse assistance, then it's time to make your own arrangements and claim back from the customer service of the responsible operator.

What is covered is mentioned in article 18 of EU Regulation 1371/2007. That's not always very specific but on the other hand, it does make clear that there are obligations.

The advice I would have given a number that have posted here is to deal with it as early as possible.

Don’t wait until late at night at a small station to expect a rail employee to solve your problem, go seek assistance as soon as you see a train delay becoming an issue.

And also expect in many cases a wide gap between theory and praxis.

Forget any food supplied by railways at all-if really extremely lucky on sweltering hot days it may be a carton of water-f.e. in DE never a bottle, as these have  deposits on them.

Best is indeed as Al suggests-and the not that many conductors in a severely delayed train may even actively go around and ask for any hoped for connections-the more people f.e. want to change to train 234 to C-the more likely it MAY wait.

Also: the more south in EUR, generally the worst of service.

In any case you can claim refunds-but with passes these are calculated on the price/day so often are very, very low. BUT you can wait till pass finished-it may be possible that with 2 or even 3 such delays you do pass the threshold.

Thanks for the replies so far - I was wondering if anybody (including Interrail) had collated these points into a simple “If your journey goes wrong” action sheet. 

So far the key thing I have taken from the answers and posts of travellers experiencing these issues (and my own earlier this year) is:

Always contact staff ASAP and/or e-mail carrier ASAP advising them you will be seeking compensation - It records the event. 

Keep calm and polite - don’t shout.

Always have Plan B and Plan C for critical trains, especially Eurostar for the return to UK - they are helpful but if you plan the last train of the day…….

Make sure you have adequate reserve funds for out of pocket expenses​​​​​​​

Keep notes and receipts at the time.

Use Interrail as a conduit for your eventual compensation claim.