The RailTeam alliance of European rail operators offers a HOTNAT ("hop on the next available train") guarantee, stated as follows:
Hop on the next available train » (HOTNAT) allows travelers to take the next high-speed service leaving from the same station as originally planned when a delay on or cancellation of a preceding Railteam member’s high-speed service prevents them from making their originally-planned connection. This service is free of charge and is subject to the following conditions : the connection that was missed must be between two high-speed trains of the Railteam Alliance. HOTNAT only applies at the station at which the passenger originally planned to change trains and in cases of train delays and train cancellations. The approval of HOTNAT is subject to the available capacity on board each train. Seats are not guaranteed.
I'm particularly interested in the applicability of this guarantee to the Brussels <-> Cologne route, where the next train is generally a Thalys if you miss your Deutsche Bahn connection. Since both DB and Thalys are RailTeam members, it seems that I should in such a case be able to take the Thalys with my DB ticket, provided that they're not completely full. (Apparently even when all seats are booked, passengers may be accommodated on fold-out seats or in the bar.) However, I've read occasional claims online that HOTNAT may not be honoured in practice -- most recently here:
Things may be complicated by the rules changing over time, and by some rail staff being unaware of current rules. So, if this does happen to me and I simply walk onto a Thalys train with my original DB ticket and a stamp proving the delay, do I risk being forced to buy a walk-on fare? Are the Thalys staff sometimes unaware of the HOTNAT provisions (in which case it might help to carry a print-out of the web page), or am I misreading the HOTNAT rule?