As I understand it, your domestic leg is only covered if:
For the former, DB will happily sell you a through ticket from (for example) Brussels to Berlin. This involves a change of trains at Koln, but as it’s a through ticket you’re covered by CIV protections all the way to Berlin (not just on the international leg from Brussels to Koln)
For the latter, certainly in the UK at least you can buy special tickets for when you’re connecting to or from an international train journey. If you’re going to get a Eurostar, or to get a Rail-Sail train, or returning from one, you can get a ticket to/from a special destination such as “London International (CIV)” or “London Eurostar (CIV)”. These tickets extend the CIV protection onto your domestic leg. They also normally have a bonus of being valid on peak time trains, but priced closer to off peak tickets, but can only be bought when in possession of an onward international train ticket (eg eurostar ticket, Dutch Flyer rail-sail ticket). See Seat61 or National Rail for more details on how they work in the UK.
Generally speaking, if you hold a through ticket with an international component, you’re covered by CIV. That could be a single international train, such as a London -> Paris Eurostar, or it could be a through ticket with a change, such as Brussels -> Berlin with a train change in Koln. A rail + sail ticket will normally also get you CIV, such as an Oxford to Dublin Ferryport, though these normally make it explicit by including CIV on the ticket. Finally you can buy a CIV connecting ticket, such as Oxford -> London International (CIV) to extend the protections onto the domestic leg.
If you don’t have such a ticket, I think you’re technically not covered (unless the country you’re in has gone above what’s required), but the train companies will often help you out all the same!
Credit:stackoverflow.com‘