Did my eyes deceive me,did I see a driver in the rear cabin?

...or c) Drivers/operators are also occasionally located in the rear cab on all train services for some reason. Odd but normal.

378 drivers explanation is the most likely, although in addition you sometimes get driver managers carrying out surreptitious assessments from the rear cab as well.

Strictly speaking the travelling to and from work is not permitted (although that's not to say it doesn't happen, especially if it's peak rush). Even when passing as part of their duty the driver in the rear cab is meant to inform their colleague driving the train. It's not completely unknown for someone to accidentally knock one of the emergency plungers in the rear cab (whilst I've not done it in that situation I have knocked the plunger in error on the second man side when putting on my hi-vis before going trackside).

I understand the cabs are interlocked so that you can't switch in one cab unless all the others in the train are switched out.

A colleague did once tell me of an incident he was involved in, in which someone coupled two Class 117s together by the brake and control connections but forgot the screw coupler. I'm not sure where the coupling up was done, presumably at Paddington although it could have been done beforehand. For some reason the lead (London-end) unit was less powerful both in engine and brake force than the other (or maybe more heavily loaded). All went well until the driver applied the brakes for the first stop (in those days Westbourne Park), when the lead unit started to gain on the other, and pulled the brake and control connections apart.

A similar dispute (without the tug of war) happened at Nottingham in 1852, when the Midland "imprisoned" a GNR loco to whose presence the management took exception.

It was apparently possible for one of the LNWR's Webb Compounds to have a tug of war with itself, thanks to independantly driven driving axles.

We are allowed to travel in the rear cab as part of our duties. Occasionally some spare drivers are required to run a test train after maintenance, they'll have a driver in the rear cab, run up to Highbury or Dalston and then change ends straight away to get out as quickly as possible.

The 378s won't allow you to 'liven up' a cab if there is a key in the other cab, although they won't always tell you. I once got caught out by this after relieving a driver at Dalston. I walked up to the platform end cab, put the key in, nothing happened and all I could hear was the passcom alarm sounding. Odd I thought, there were no BIL lights flashing. I keyed off and on a few times, same thing. I then walked down through the train to check all the alarms and it wasn't until I got to the rear carriage that I could hear the fault alarm sounding in the rear cab. The other driver had left the key in and train on, obviously distracted by my presence when he pulled in.

There are numerous stories (many exaggerated) from BR days of diesel loco jumpers being wired up internally "back to front" so that when they were multipled with another loco they pulled in opposite directions.

I am aware of a similar problem with HST power cars in the very early days on the WR caused by sticking contactors although they didn't pull in opposite directions, one just acted like a mobile load bank and presented so much rolling resistance that speed would not exceed about 30 mph.

On an LUL battery loco I have had one set of motors trying to go forward and the other set in reverse when I opened up the power controller. The loco just sat there juddering with a bit of wheelspin!