Ultimately the objective should be a system similar to that used in America for locomotives (the AAR system), where essentially all diesel locomotives built for the last 70 years can operate in multiple with each other.
Such a system was appearing before privatisation, with essentially one DMU, one EMU standard [at least for PEP/Mark 3/Networker], and moves towards a standard TDM system for locomotives.
Privatisation effectively killed all that.
The RSSB has done studies on coupler standardisation and concluded that the benefits of full electrical compatability would be "high", but stopped short of a full quantitative analysis, although they certainly believed mechanical standardisation was worth pursuing purely from a rescue perspective.
The document does state the following:
"Standardisation of coupling arrangements will bring a major reduction in the time failed trains take to be rescued: it is estimated that delays due to failed trains will halve, saving the industry money. Other benefits include safety benefits for drivers due to the avoidance of accidents from manual coupling, reduced depot maintenance costs from using just one type of coupler, reduced purchase cost of couplers due to competition, cascade benefits from improved use of old rolling stock and reduced operating costs from better utilisation."
One of the paragraphs summaries what types of trains should have what standard of coupler, in this case the couplers are all Dellner couplers:
"A two-tier approach to standardisation is recommended – the Dellner Type 10 coupler, which is already mandated by a Technical Standard for Interoperability for trains with a design speed of 250kmh (155mph) or greater, and the Dellner Type 12 and compatible Voith Type 136 coupler for trains with lower maximum design speeds. This design is known as the Standard Latch Coupler (SLC). A cost model was created which concluded that the estimated costs of retrofitting the SLC coupler to existing vehicles was between £6,500 and £15,000 per vehicle end. The higher costs are for small fleets like the Class 155 Diesel Multiple Units where design costs are spread over fewer trains.
The only regulation preventing adoption of the SLC as a standard design for GB Rail was the TSI which says trains capable of 225 km/h should have the larger Type 10 coupler. As this is only likely to cover trains on the largely free-standing High Speed 1/High Speed 2 routes, it does not materially affect the business case for equipping all other passenger stock in the foreseeable future."
As we don't have many trains with the design speed of 155mph, I guess all trains would be using Dellner Type 12 couplers? So you could have and I am saying this tongue in cheek, a two car class 155 rescuing rescuing a Nova 3 set!
I will leave others to work out the cost by type of locomotive and unit per TOC etc... it would cost to make the suggested changes.
But I have to agree with the comment below:
Rather than trying to make everything work with everything all the time, would it not be better to think about how we can make them work together when it's most needed which is usually when something goes wrong?
Things like ensuring that suitable, easily assembled and readily available emergency couplers, making sure that said couplers don't take hours to get to site and that there are suitably trained staff close by to use them and having the right processes in place to make this all work might be a much better idea. It might also be easier to achieve some form of interoperability if we focused on allowing as many types of locomotive and unit as possible able to provide basic assistance such as lighting power and heating/aircon power when it goes wrong. Essentially having the interoperability when it matters but accepting that some of the nice to haves may not be possible.
