There were several factors that prevent the insertion of BR Mk 1 hauled stock into a dmu formation with the gangways connected, these are :
- Incompatible gangways
- Vacuum pipes
- Multiple unit controls
The gangways are well described above. Class 101 and similar dmus used the simpler, older type gangways, whereas Mk 1 stock used "Pullman" gangways. These had indeed originally appeared on Pullman cars pre-war, as they gave a decidedly more weatherproof and less intimidating connection without the view of the ballast flashing past below, but had spread to SR and LNER stock. You can actually connect the two types using adapter plates, providing you have any to hand, which can be a bit of a palaver to install. This didn't particularly matter if you tagged the coach on the back, behind the rear driving compartment, with its gangway locked.
Mk 1 stock had a standard vacuum brake pipe, whereas dmus typically had a second pipe which was used as a "quick release". This prevents inserting the coach in the middle of the formation as the dmu brakes to the rear would not work well. However, a vacuum vehicle on the back of the formation with only the main pipe connected does work, albeit taking longer to release.
The MU control connections for throttle, engine starting, reversing direction, etc, are an absolute no-no for inserting a non-MU vehicle in the middle. The best known was the "Blue Square" system, although the others are mostly variations on this theme. Lack of these controls however doesn't matter if your extra coach is attached at the back of the formation, although this then becomes a real nuisance and requires shunting at the end of the trip.
Done often? Well it was more so in early dmu days, then seemed to die out, in no small part because the always poor power/weight ratio became worse with an extra unpowered vehicle, and Mk 1 hauled stock was much heavier than dmu cars which used lightweight construction techniques. The WR seems to have been more into it than most; when the Birmingham Snow Hill-Hereford-Cardiff service went over to Cross Country dmus it was common to have a 6-car formation (four motor cars) and attach an extra coach on the back. Still must have been a grind up through Malvern, and they normally used an ex-GWR corridor coach, that were lighter than the BR Mk 1. The WR then did a bit more, and converted several ex-GWR corridor coaches to proper mid-DMU trailers, with all the connections described above installed properly. They had done this previously with coaches inserted into the twin-sets of the original GWR diesel railcars of the 1940s. Much later the Western took some surplus dmu power cars, stripped the seats out, and inserted a GUV (again properly converted) in the middle, using them on parcels and mail operations - which occasionally might multiple with normal passenger dmus.
