For my part I was guilty of an over-simplification in my OP▸ . The following is a moderately technical explanation which goes part way towards explaining the rule.
Firstly, the physical barrier presented by non-gangwayed stock means that some local authorities prefer a guard in the trailing set to cope with any unforeseen circumstances. As such, there will be a train crew member in each portion of the train. I should however emphasise that current safety features are tried and proven and that the positioning of guards is an 'extra layer' as it were.
For example, unit doors are so wired that brake release cannot be obtained until all are shut. (TIS‡ or Traction Interlock Switch). Similarly, should a door open in traffic, the train will be brought to a halt. This can be bypassed when dealing with faults that would otherwise incapacitate a train. When the TIS is isolated, brake release can be obtained without doors being closed and doors can open in traffic without the train being halted. In such cases, the guard will need to make sure that all doors are held closed by air pressure following every stop. The driver would do this in leading unit and the guard in the trailing portion.
In another scenario:-
In normal circumstances, an electrical circuit, (Train Wire) runs around the train, passing through the various train brake circuits and the handles in each cab. The Train Wire draws power from the rearmost battery in the trailing vehicle. Break the circuit, (perhaps by division of the train) and brakes are activated throughout ALL portions of the train. (Activation of the Passcomm also breaks the circuit so stopping the train). So far so good.
However, there are very rare fault-based occurrences which permit drivers to isolate this system. Rules state that pax must be evacuated from rear unit if practicable, (difficult, dangerous and not a viable option with non gangwayed stock when not platformed). The guard must then ride in the leading cab of the trailing unit to apply the handbrake, should the train divide, as the train wire has in effect, been 'switched off.' Thus, all remotely controlled brake handles, (normally 'live' for emergency brake application), are rendered inoperative, excepting that in the cab from which the train is being driven.
Obviously in above circumstances, the train would be taken out of service at the first practicable location.
I totally empathise with Inspector Blakey's viewpoint ref interconnected units though and cannot offer a better explanation. I can only surmise that a guard in the rear unit is viewed as a means of maximising the benefits of an already considerable safety system.
/
Almost there, but you are using the wrong termonology and wrong operating princicapals.
The train wires, of which there are 42 on
DMUs▸ , are resonsible for through-control in a multiple-unit formation, wether there are two vehicles or ten. The train wires (TWs) are passed between vehicles via the autocoupler contact boxes as found on the driving or cab ends of DMU vehicles, or by 42-way jumpers and recepticles, as found on the intermeidiate or non-driving ends of DMU vehicles.
Each trainwire is invidually numbered and has a singal use, except the lights on/off TWs, which are also used to transmitting
PA▸ signals, and several are spare or nt=ot used. As you correctly state, the logic of the trainwires is that the rear vehcile of an electrcially joined multiple-unit formation provides the feeds through the trainwires.
Now lets adress the errors!
The TIS (traction interlock switch) is a normally sealed switch that enables the Traction Interlock circuit on the vehicle in question to be overridden, allowing
traction power to be obtained despite the loss of interlock either through a fault or a door not being able to be shut properly.
Traction Interlock is TW36 and passes through only two relays per vehicle which make/break the trainwire depending on the state of the inerlock on that individual vehicle.
Brake release is determined by TW4 'Brake Continuity', which relies on several crucial interlocks, including suffcient air pressure to control brakes (Main Resovoir Governor), correct coupling of both of the vehicles autocouplers (through a microswitchs operated by the movment of the coupling pin and a couple-proving relay), Drivers brake controller (if the brake controller in any cab in a unit formation is put into Emergancy, brake continuity is broken) and a couple of other switches and relays.
The electrcial systems interconnecting with the trainwires are often very complex and practifcally fail-safe, for example, Operating a Passcom for example does not directly break TW4, but instead breaks a circuit that deenergises a relay called PER (Passenger Emergancy Relay) that then brakes TW4 and thus removess brake continuity and gives an emergancy brake application.
Like the TIS for the traction circuit, there is also an override availiable for the brake continuity circuit, called an
EBS▸ (Emergacny Bypass Switch). This is a get out of jail switch that can be operated in the event of a fault conditon or TW4 fault that prevents brake release. Operating this flag switch in the driving cab, removes the main function of the trainwire, and renders the brake controllers in other cabs, the Passscoms and the coupling interlocks redundant, but still retains the MRG (Main Resovoir Govorner) protection so that the brakes cannot be released if there is not suffceint air pressure to re-apply them.
I must emphasis that the EBS is what its name states, an emergancy last resort and as the OP correctly stated, there are specific conditions in which the EBS and also the TIS can be operated on a mainline service train and strict procedures that must be followed by the traincrew while the EBS/TIS is operated, such as sagain the OP has given examples of.
Hope this expliand things a litte bit and i have not chucked too much technical stuff in.
Click on the map to explore geographics
