Some trains have different couplings than other trains such as BSI, Tightlock, Screw (Common in older Mk1 Stock and Modern Locos). When moving a train with incompatible couplings i.e. both have different couplings then you can do two options.
Move the train with an emergency coupling. These are only used in emergencies (Hence the name) to usually drag a failed train away to clear the line. This is just a simple coupling all trains have in one form or another. HSTs and Pendolinos both have them through movable front panels. The problem is that some trains can't have a through brake in one form or another. In other words the train only has a brake on the Locomotive or lead unit. So it can't be effectively moved long distances.
Another option is a Barrier Coach. Usually a small Mk1 converted coach with specialist couplings on such as BSI, Tightlock, Dellner to attach the unit. And the other end have similar things. The Barrier Wagon is capable of providing a brake for the whole train therefore making it effective to use on Long Distances. They are usually found on trains that go through the Channel Tunnel e.g. 450s/350s/185s. An example train would be a 92 (Screw) + Barrier Wagon (Dellner coupling) + 185 (Has a Dellner Coupling).
These wagons can usually be found in Wembley Yard as that's where International Trains with New build units end up.