Is part of the problem that the units, since the new timetable are running more miles(Or being used for longer during the day)? resulting in more accumulated breakdowns? ( I don't know if they run more miles now since May , just asking). I guess on a double track line with a failed unit, its the worse possible place to breakdown, as you can't "go round it"- amazed just for a brake pipe fault it took over 2 hours to get it moving, couldn't a preceding or behind unit just buffer up to it and take it past the double track section, so that's its only blocking one line out of 4? However I bet all the different units don't have compatible couplings? could a 365, tow a 313 or 700 or vice versa? What about the alleged "Thunderbird" loco at KC? One fault anywhere on the Thameslink network - north or south of the river seems to create meltdown across all of it.
Just to point out a few things:
1. Brake defects can take time to resolve depending on what the actual fault is.
2. Even if said unit was available, there's no saying it might transfer the fault to the assisting unit as has happened in the past and will in the future.
3. Said unit might not even be available.
4. A Class 313 and a Class 365 do share the same coupling type so can couple mechanically as can a Class 313 to a Class 317 etc, I believe they use a emergency braking hose which is kept in the vestibule of the Class 313 however the Class 700 as well as the Class 387 has a different coupling type to the Class 313/365 so would need a emergency coupling and braking hose.
5. Said equipment might have to come by road and the team could have been held up in traffic, it's not like they can have flashing lights to ensure people move out of the way.
6. The Thunderbird at Kings Cross would only be useful if LNER had a driver available and GTR were willing to pay LNER for the hiring of the Thunderbird and Driver, remember they are different companies so unless LNER got paid for the hiring of their resources they're unlikely to do anything.
7. It also depends on where the train broke down as that could make access very difficult for these trying to resolve the issue.