D78 Stock - Coasting and Weak Field Controls
A question which comes up often about the D78 Stock is 'what are those little coloured panels that can be seen in the cab window?'
The answer is that they are to do with the control of speed and acceleration rates of the trains. They are in fact switches - we refer to them as 'Flag switches' or just 'Flags'; if raised they indicate that a particular control choice is selected.
The following illustrates these:
The purpose and the effect of each is as follows:
- If both 'Flags' are down (that is they are not visible) the train is limited to 30 mph. Even if the Train Operator keeps the Combined Traction Brake Controller (CTBC) in a position where it is calling for current to be passed to the motors, on reaching 30 mph the motors would 'cut out' thus keeping the train to this speed; the train would coast. Once speed reduced below 30 mph the motors would cut back in until 30 mph was regained.
- If the 'Weak Field' Flag is raised (visible) the acceleration rate is slightly increased and the train will now achieve 40 mph before the 'coasting' effect as described above takes over. There are only certain areas where this is permissible - more on this later.
- If the 'Coasting Control' Flag is raised the train has the lower acceleration rate but will continue to gain speed until the Train Operator returns the CTBC to a position where motoring is not being requested. Potentially the train will get to about 50 mph in this configuration (space permitting!).
- If both Flags were to be raised the train would would have the potential to reach its maximum design speed - about 60 mph. However this configuration is not permissible - see below.
So, when are these options used?
The timetable is designed round the following criteria:
- That the Weak Field flag will only be raised (therefore visible) west of Turnham Green , west of Putney Bridge and East of Bow Road.
- Between these areas both flags should be lowered (not visible) except that in the event of late running the Coasting Control flag may be raised so that some time may be recovered. The speeds attained by the train must however not exceed any posted speed restrictions, either temporary (TSR) or permanent (PSR).
- The Coasting Control may also be raised to overcome certain 'speed sensed' defects on the train.
Many Train Operators prefer however to run with the Coasting Flag switch raised at all times along the length of the line. This means that should this option be their preferred choice of operating the train the Weak Field Flag would not be raised at any time - see below.
The following is a quote from the District Line Supplement on the matter of 'Use of Flag Switches ('D78' Stock'):
'Use of the Flag switch is restricted to:
- east of Bow Road
- west of Turnham Green
- west of Putney Bridge
Use of the Flag switch on all other areas of the District Line is prohibited.
The Coasting Control flag switch can be used on all sections of the District Line. Simultaneous use of both Flag switches (i.e. Use of Flag and Coasting) is prohibited.'
The reason for this last restriction is that if running at maximum speed and the Train Operator was to overrun a signal at danger there is a theoretical but minimal chance that the train could (although it would have course have been 'tripped' and the emergency brake applied) collide with a train in the section ahead.