These were an updated version of the 79xxx Swindon Inter-City vehicles. They retained the two cab design (full-width 'leading' and half-cab 'intermediate'), but one big change was that a guards van no longer had to be at the outer ends of a set. This allowed leading power cars to have the van at the rear of the vehicle and passengers being able to share the driver's view of the line ahead (providing the blinds were left up!). The other change to the DMBS was a centre passenger door meaning two passenger saloons rather than one. As there was no longer a need for the van at both ends of a train, the intermediate DMBS type was no longer needed, and the layout of the 79xxx DMS was retained. The buffet cars and TFs were also the same, but there was also a TC type introduced. To avoid increased costs, these kept the seven compartment arrangement (meaning the same body construction jigs could be used). This meant that the size of second class compartment matched that of a first, so there was more legroom than normal in second class compartments.
The cars were of steel construction and featured Pullman gangways and buckeye couplings. Each power car was fitted with two AEC 150h.p. underfloor engines with mechanical transmission giving a maximum speed of 70 m.p.h.
This view of a DMS in Ayr depot in 1977 shows the details on the inner end of a power car. All Swindon DMUs had grilles fitted over the exhaust pipes here. One reason for this is as with Mark I coaches they originally had footsteps up the rear; the grilles would prevent staff from coming in contact with the exhaust climbing the steps. The footsteps didn't last long, the upper three were removed - their position is where the sets of four studs can be seen. Originally there was a stepboard above the gangway which was also removed leaving the four brackets behind. The gangway top cover is suspended from the inner two of these brackets. Above the footstep which remains is a cast plate which carried the vehicle dimensions, another Swindon DMU peculiarity. On the DMS the toilet was at the front of the vehicle, and this gives the inner end of the DMS one of the few differences from the DMBS; on the DMBS the water filler pipe acts as a handrail when climbing the steps, on the DMS this is purely a handrail, identical in style, with an extra grab rail on the roof. Vincent Haworth.
There were five vehicle type in the 5xxxx batch:
The cab fronts were very similar to the earlier batch, the only changes were that the stencil train indicator was replaced by the now standard 4-character route indicator. The 'leading' type power car had this split into two parts with a gap in the middle - the cab front has a slight angle towards the centre and this method allowed a flush finish and avoided adding a box to contain it which was done on the final batch of Class 120s. The 'intermediate' power cars had a two-character box either side of the gangway. Another difference from the first batch was that leading power cars now had jumper connections fitted.
Like the earlier 79xxx vehicles they had the White Circle control system, yet it was substanially different from the earlier system. It was entirely a blue square system, except the wires that went to the end jumper connections were in a different order. This was done to match the connections on the earler vehicles, which was based on the yellow diamond system. Again, there was no through air pipe connecting the vehicles, each power car solely relying on its own control air supply.
The 1970s saw the first class accommodation downgraded. Being of a non-standard design, the class was not included in the DMU refurbishment programme so remained in close-to-original condition. The main alteration was the plating over of the outer gangway connection on the DMS vehicles in 1979-81 owing drivers' complaints of draughts.
As the 4-character headcodes became obsolete these could be found replaced with white dots on black. In later days the gangways were removed from the cab end of the intermediate power cars (often done at Polmadie depot), the gap being plated over and painted yellow. The image shows both these alterations on DMS 51026. Note the bottom of the gangway rubbing plate has been left on as it carries the lamp brackets, needed to carry the tail lamp. Andrew McConnell