Design and construction
Two underframe lengths were specified for the BR Mark 1 coach, 57ft for suburban and parcels stock and 64ft6in for mainline corridor stock. One critical factor was the inside wheelbase, which was set at 38 feet to ensure the new, longer coach would still block point locking or fouling bars which were assessed as being or to be brought up to a minimum 40ft length. (The fouling bar has to be raised before points are unlocked and will be prevented from doing so by being 'fouled' by the wheel flanges, ensuring points cannot be unlocked while a train is passing over them.) Coaches with the chosen combination of length and bogie centres could be accepted over all routes, with just a few restrictions on minor branch lines, dock and industrial sidings.
Applying the modular design allowed both these underframes to be assembled using common end units incorporating the drawgear and bogie pivot centres, with two central sections being omitted to assemble the shorter 57ft frame. The underframe was generally stronger than any previous design and being designed for use with central Buckeye knuckle couplers featured two substantial central main truss beams. Retractable side buffers and a standard drawhook were fitted to allow coupling to locomotives.
The body was built from a range of standard side panel units comprising doors, windows (wide for compartments, narrow for toilets etc.), compartment wall panels (between window openings) and corresponding 'blank' panels. These could be pre-assembled complete with stiffening ribs etc. ready to be welded into a complete bodyshell. This allowed many of the components and panels to be mass-produced on common jigs and fixtures then stockpiled, ready for assembly. Different types of coach bodyshell (eg. second opens vs brake seconds) were built by simply assembling the appropriate standard body panel sections into the order required.
The standard corridor stock end units featured the wide Pullman gangways and were bowed outwards over the headstock and coupling, increasing the vestibule area and shortening the length of the gangway. The gangway floor housed a sprung pressure plate to keep the buckeye coupling beneath 'stretched', controlling the slack in the couplers between coaches and keeping the gangways pressed together. Coach interiors were constructed in wood, almost entirely screwed together and, as everything was built to standard dimensions, much of the interiors could also be pre-made, ready for assembly inside the bodyshell. Later built and refurbished Mark 1 coaches were fitted with melamine surfaced panelling and flourescent lighting in place of the original tungsten bulbs, giving a much brighter if less traditional interior.
These methods greatly speeded up the construction of the underframe and bodyshell and the fitting out of the new coach, and have proven highly advantageous in the maintenance and repair of these coaches. Damaged parts can often simply be unscrewed and replaced with new or refurbished parts from the stores. The design has also proved easy to adapt to meet new requirements, much of the Southern region EMU stock from the 1950s used Mark 1 design vehicles and many engineers vans for breakdown trains were converted from passenger stock with extra door units replacing seating bay windows. More recently heritage railways have adapted coaches for disabled access by fitting double doors in the vestibule, giving access to the end seating bay for wheelchairs and accompanying passengers.
Bogies
Th most noticeable variation between Mark 1 coaches of the same type is the range of bogies they ride upon.
The original BR mark 1 bogie (BR1) is a traditional riveted plate frame design with plain bearings based on the late 1930s designs from the GWR and LMS. The double-bolster arrangement used for mainline stock was intended to provide a smooth ride at speed, with suburban stock which wasn't expected to travel at high speed being fitted with a similar but simpler single bolster version.
The BR1 bogie was satisfactory when new but was found to wear more rapidly than had been anticipated, causing the ride experienced by the passenger to deteriorate as the bogies accumulated miles long before reaching the planned shopping mileage. The BR1 bogie also performed poorly under the heavier kitchen/diner coaches, resulting in older Gresley design bogies being substituted. A number of alternative bogie and suspension designs were tested under the coaches, including designs from European builders.
The heavier English Steel Castings cast steel drop-equalised Commonwealth bogie clearly out-performed other options and coaches built from 1958 were fitted with these bogies, ushering in the use of roller bearings as a standard fitting.
The faults with the BR1 bogie were eventually traced to the variable quality of materials available in the immediate post-war period (compared to 1930s materials) and faulty riveting during assembly resulting in the axle guides not being as tightly fixed as might have been anticipated. However by the time the issues had been identified and corrective actions formulated the far more modern B4 (mark 4) bogie was under development.
Designed for running at 100mph the B4 bogie proved to be one of the finest high-speed passenger coach bogies in the world during the 1960s, when many European railways considered exceeding 100kph (60mph) to be fast! The B4 bogie became standard for the next generation Mark 2 coaches built from 1964 and was fitted to late-build Mark 1 coaches. Many more Mark 1 coaches from earlier builds running in express passenger services were also equipped with the new bogie, which was a lighter alternative to the Commonwealth type.
Due to its versatility and superb performance BR required the return of B4 bogies from vehicles going for scrap or sent an exchange set of BR1 bogies for coaches going to heritage railways until the late 1980s, fitting them to stowage vans (BGs) used on express parcels and Travelling Post Office services. Since 1990 several heritage railways have purchased both Commonwealth and B4 bogie sets to replace BR1 bogies on their Mark 1 coaches.
Experiments and developments
In 1957 a few experimental coaches were built to test new ideas and improve on the existing design. Passenger comfort may have improved but in most cases passenger capacity fell, resulting in a lower passenger-per-ton figure, not appreciated by the accountant! One experimental fibreglass body was constructed at Eastleigh using a salvaged underframe and another body was built using Leyland National bus body pressings as a possible rebodying option for coaches suffering severe rot. While none of these experiments were proceeded with several have been preserved and the Leyland National bus body led to the development of the Pacer and Sprinter trains which were to replace Mark 1 coaches on secondary duties.
The final experimental train was designated XP64, a set of 8 coaches built with a new body style and several features for in-service testing prior to the building of the Mark 2 stock. The XP64 coaches retained the Mark 1 construction however, having a strength underframe carrying a separate body. The revised body styling incorporated a skirt over the visible underframe solebars, the final Mark 2 bodyshell was designed as an integral structure, the underframe members forming the floor and the body 'tube' delivering additional strength to the bodyshell.
Modifications, service and preservation
In addition to the issues with the bogies the Mark 1 coach suffered a few other defects in design or from service use.
The bodyside was formed into a continuous curve, unlike other coach designs before and since which have had flat sides along the window line (usually canted inwards at the top) and a 'tumblehome' below the waistline, giving a maximum width at about seat height. The window glazing remained flat however. When new the windows were well sealed, but rapidly began to allow water to seep into the body. While the water ingress was addressed by the fitting of external aluminium window frames on later builds patching strips along the bottom edges of the sides were a common repair.
Later builds of coaches were also fitted with cast aluminium doors, which again resisted rusting much better than the steel-sheathed originals.
The Mark 1 coach was designed to have service a life of 25 to 30 years and this must in general be considered to have been successfully achieved. By the mid-1970s new rolling stock and trains were coming into service and Mark 1 coaches had been removed from the principal expresses, but still provided comfortable accommodation on secondary and cross country trains. The relative safety of the Mark 1 compared to newer designs was also now coming under scrutiny, however even after the 1988 Clapham collision the official report concedes that, although not providing the collision protection of later coach designs, the 1950 Mark 1 coach design was not
inherently unsafe, possessed good riding qualities and was not intrinsically
lacking collision resistance.
However the coaches were approaching the end of their design lives, so deterioration can only be expected and coaches suffering severe problems could usually be withdrawn from service. British Rail was already progressing the full replacement of the locomotive hauled Mark 1 fleet by then, with the class 156 and 158 express Sprinter trains replacing many locomotive hauled trains the major hold-out was the extensive fleet of Mark 1 EMU stock on the Southern region. A target date of 2002 had been set for the removal of Mark 1 coaches from the national network as these were considered to be 'less crashworthy' than newer stock. This was subsequently postponed and modified, as new trains simply could not be built fast enough to achieve it.
The 1999 Paddington collision in which a newer 'crash-worthy' DMU was utterly destroyed HST refocused attention, highlighting that instead of worrying about crashworthyness of old trains, it would be better if the trains didn't crash in the first place! The resulting rapid roll-out of the Train Protection Warning System (TPWS) has mitigated many risks.
A small fleet of Mark 1 stock remains registered with Network Rail, used mostly for tour trains hauled by steam and heritage diesel locomotives and not in regular (daily) service. Most of these coaches are from the later build batches and have been fully overhauled, restoring their designed safety features, particularly the collision posts which are prone to collecting condensation and rusting through. The coaches are required to run in trains equipped with TPWS working and often the operator will provide a service coach at the outer end as a further protective barrier. As Network Rail now requires TPWS in all operative cabs all mainline certified locomotives now meet these requirements.
The modular construction methods have made repairing the Mark 1 coach very easy. The interior can be dismantled, giving access to the body panelling. Use of steel throughout the body makes weld in new ribs or sheeting straightforward, plus the modules allow easy modification and with a wealth of knowledge of the Mark 1s faults, these can readily be cured!
Mark 1 coaches form the backbone of the passenger fleets of Britains heritage railways and following through repairs passengers can be confident preserved Mark 1s will be a safe and comfortable coaches for another 50 years at least!