Bulleid's 4-DD 'Double Deck' Electric Multiple Units.

Although strictly not a locomotive, this innovative idea arose during 1948, in the months prior to nationalisation, as the Southern sought to carry more people on crowded rush hour suburban routes.  O.V.S. Bulleid had plans drawn up for the boldest of British EMU design - the resulting 4-DD sets were actually the only 'double-deck' trains to run on the British main line, holding 122 more passengers than a standard 4 car unit.

                                                    
Based on the successful Southern Railway Suburban electric multiple unit design, the slightly wider and taller bodies had passenger compartments alternating between two levels.  By reducing the floor level of the passenger area by 6", and increasing the roof height by 4½", enough extra depth was gained to squeeze in the stepped compartments.  The upper seats occupied the relative position of the luggage rack of the compartment below, each 'high' deck being accessed by narrow central steps from the lower compartment and the same door; so not a 'double decker' in the strictest sense.

These 'slam door' units were built just after Nationalisation, outshopped from Lancing Works in late 1949.

The eight carriages featured new innovations for S.R. EMUs such as slim strip lighting improving the interior illumination, needed because of low ceiling heights.  There was also pressure ventilation, necessary as no opening windows were fitted on the upper deck compartments due to the proximity of the loading gauge - even so, the upper levels became uncomfortably hot during warm weather.

As the 4-DD units were much taller than the regular loading gauge, they were restricted to operating only the suburban lines between Dartford and Charing Cross.  To keep within the maximum permitted width, no external stepboards were fitted, resulting in a big gap between the train and the platform.

                                                    
Each four car unit could seat 552 passengers, the driving cars had five upper and lower compartments accommodating 55 passengers on each level.  The trailer cars seated 66 on the high level within six compartments and on the lower 78 within seven compartments.  A further 150 standing passengers could be carried.  Also on the high level, tip-up seats were positioned under the large middle window.  The upper windows were of curved glass which ran up to the roof line.

Power bogies were mounted on the outer ends of the driving cars, each pair of traction motors driving 3' 2" diameter wheels.  The shorter 8' trailing bogies fitted under the lower passenger floor sections had 3' diameter wheels.  Power and control equipment was by English electric, and included electro pneumatic brakes.  The units were originally constructed with welded assembly BFB wheels which soon suffered fatigue problems, and were replaced by standard wheels.

A good concept, but in reality the upper compartments were crampt, and with a standard width single slam door entry, the loading and unloading times were longer than for a standard carriage.  As a result, they did not attract passengers in the peak hours, and seats often remained empty on the upper area.  There were also passenger security issues, young women especially felt they were too trapped in the upper compartments.

Turnaround times at stations were increased because of the increased number of passengers per door, and the two tier layout proved generally unpopular.  Instead, to reduce overcrowding and obtain the extra seating capacity required, the S.R. decided to lengthen platforms and adopt standard 10 car trains.  Both of the 4-DD units stayed on in service until withdrawn on 1st October 1971, the last service being from Charing Cross to Dartford.

When constructed, the sets were painted in BR green livery, numbered 4001 and 4002, although they were usually paired to form an 8 car unit; in the late 1960s they were repainted in BR rail blue.

After they were withdrawn, a commemorative plaque was erected at Dartford station.

These units were often referred to as an experiment, but the fact remains that they were in active service for over 20 years.

I have heard mixed reports about travelling in these units, but I think if you were a commuter for whom these were really designed, they were a poor method of travel.

The accommodation was quite cramped, and access up the steps from the lower compartment was difficult in crowded conditions.

To quote one passenger:  "I used to travel up from Blackheath and they were used occasionally on my morning train, awful things to try and get out off at London Bridge if it was crowded and you were upstairs.  It must have been one of the last trains to have a 'Ladies Only' compartment - not that anyone took any notice in the rush hour !

And another:   "I used to catch the double decker train every morning and evening from Slade Green.  It would come out of Slade Green sidings and wait on the down platform.  It would then go round the loop and on the Barnehurst line.  I loved this train.  I particularly remember the Ladies Only compartments.  Frustrating if you were late and had to hunt firstly for a non smoking compartment and evading the Ladies Only carriages.  It took an age for people to get on and off when full.  If you happened to be upstairs in a smoking compartment you came out kippered."

{   Unit Numbers     DMBT       TT         TT       DMBT 4 Car Formations   {   4001 (later 4901)     13001 + 13501 + 13502 + 13002    (built September 1949)  {   4002 (later 4902)     13003 + 13503 + 13504 + 13004    (built October 1949)

                                                    
Driving cars 13003, and 13004 have survived scrapping, but are under different ownerships.
13003 is kept at a private location in Ashford, Kent, - and said to be in a deplorable condition.

13004 is at the Northamptonshire Ironstone Railway Trust, where some restoration work was done a few years back.
One trailer car was intially rescued after withdrawal, but has since been scrapped.

It is perhaps sad that this unique piece of British Railway history was not adopted by the N.R.M. and cared for under cover.

For a while, a 4mm scale model resin based kit was available of the 4-DD for modellers, supplied by the "The Engine Works" of Canterbury, though only 25 sets were made.

There has been some recent discussion about re-introducing double deck trains to the UK, common in Europe and North America where there is a more generous loading gauge.  In the U.K. work to raise bridges and deepen tunnels (as has already been done to to accommodate taller shipping containers) could prove less complicated than that required to introduce longer trains, which can interfere with signalling equipment and block junctions near stations.

If this ever happens it is likely to be in the busy commuter routes south of London; attempting to resolve exactly the same problem as Bulleid's team were addressing - crowded trains and short platforms.  There was no enhancement to the loading gauge to accommodate Bulleid's trains, and sadly they were unacceptably cramped as a result.  In the roughly 45 years since these units ceased running, the average size and height of the population has increased, and it is difficult to see how [ within the same relative loading gauge ] a modern solution could really address the problem any better.