It is a huge issue to deliver the principle of "everything must couple to everything else". A lot has happened since the old Southern buckeye couple, 27 way jumper and twin air hoses. There is a very large number of functions for which there are no UK let alone international standards. Indeed even simple things like relay logic train wires can cause problems because some trains use 24 Vdc and others use 110 Vdc. On the old BR provincial fleet, the interfaces were very simple - along the lines of the old SR days, and it was easy enough for Turbostar to be specified to couple to them. With more and more in-board kit, and fitting things like ETCS it will become increasingly difficult especially with the data systems that send signals up and down the train. I doubt that any of Northern's new trains will couple to the legacy fleet except for recovery.
Let's assume folk put in the effort to create a modern standard: "this coupler at this height will be used from now on and this is the list of functions and their specifications that have to pass across the coupler". What happens the following year when someone comes along with a new function that no one has thought of before? Is that function not allowed, or do those new trains become the exception? And what happens the following year with yet another new function?......and so on. I hope this illustrates why it's hard.
