Nick Leverton wrote:

I believe trip cocks were previously fitted to the DMUs on Widened Lines
services, and they seemed to work mostly. I also read on the web that
some (presumably Sulzer) type 2s were fitted for these trains before
the DMUs took over, and also for other LTE workings. Any more gen on
either would be welcome :-)

Both the LMR and ER had locos and DMUs so fitted.

The LMR had some Sulzer Type 2s, plus torque convertor DMUs. The ER
had the Rolls-Royce engined DMUs, plus Brush Type 2s. The Brush 2s
lasted until the closure of the route to Moorgate via York Road, when
the GN electrics started via Drayton Park, and the LMR DMUs lasted
until the Bed-Pan services started to run to Moorgate. The tripcock on
GN suburban workings (originally Quad-Arts, later BR non-corridor
stock) was set by the Driver's Assistant at York Road, but on many
occasions the isolating cock (let into the skirt under the nose)
wouldn't be opened, leaving the tripcock merely to satisfy the LT
signalman via the tester at what is now KX Thameslink.

The loco hauled services worked on the basis of a released loco taking
the next train from Moorgate, with a light loco going down there at the
start of each day. Coupling/uncoupling was done by an LT member of
staff.

ER BTH Type 1s were also fitted with tripcocks, for working freight
over the line taken over by the Northern Line to High Barnet. These
carried short-circuiting bars on these duties, but trains working over
the Widened Lines didn't, as they didn't go over any conductor rails.