Saw that the Llangollen Railway was running its Class 127 and 109 in multiple today. In BR use they had different coupling codes, because the 127s had automatic (hydraulic) transmissions and most other classes (including 109) are mechanical (manual). How have they overcome this problem? As far as I could tell the 109 was changing gears despite the 127 leading.
As Bill57p9 says, the 127s were designed as new to multi with Blue Square mechanical units.
They have a gear selector handle, just like mechanical units but 4th is labelled 'D' not 4th. When running a fully hydraulic formation, this is left in 'D' with the unit(s) acceleration fully automatically, changing from torque converter to fluid coupling at approximately 46mph.
When running a mixed formation, the selector should be used to change gears as per normal mechanical driving technique, any 127s in the formation will continue to operate automatically changing to direct drive at 46mph as before.
What happened in service was drivers forgot to operate the gear changing sequence when driving from a 127 with a mechanical set on the back, cue many damaged gearboxes.
Consequently, a paper change to the coupling code on the 127s was made but I don't believe any physical changes to prevent multiple operation between 127s and Blue Square mechanical sets were made.
In preservation, so long as anyone is careful to observe appropriate driving techniques in a mixed formation, there's no problem with running a mixed 127/109 formation as per the Llangollen example you give.
