Editing Advanced Passenger Train (section)

===Design review===
With the decision to move primarily to electrification made in November 1972, Jones began building a larger management team to carry the design forward to service. This resulted in the April 1973 transfer of the design from the research division to the Office of the Chief Mechanical and Electrical Engineer. A review was carried out by a joint team from the two divisions, led by David Boocock.{{sfn|Gilchrist|2006|p=35}}

As a result of this review a number of additional changes were made to the design. A major problem was the recent discovery that the overhead lines on the WCML were subject to the creation of large waves in the lines at speeds over {{convert|200|km/h}}. This was not a problem for two trains following each other with a spacing of several kilometers, but was a serious problem for a single train with pantographs at both ends. The obvious solution was to use a single pantograph at the front or back and then run the power between the cars, but this was outlawed by concerns over the presence of 25&nbsp;kV power on the passenger cars.{{sfn|Gilchrist|2006|p=35}}{{efn|Sources disagree on the nature of the problem of running power along the train. Wickens states this was a safety concern,{{sfn|Wickens|1988}} while Williams states it was due to the difficulty of designing a coupling between the cars that could handle the case of the two cars being at different tilt angles - only the center of the cars would remain at the same relative alignment, not the top or bottom. These may not be different issues; because the only point that was guaranteed to be at the same angle between two cars were the shared bogies at either end, the power cabling would have to either run under the cars, or from the roof down to the bogies and back up again repeatedly, which would carry the cabling though the passenger compartment.}}

Some consideration was given to placing both engines back-to-back at one end of the train, but concerns were raised over excessive buckling forces when pushing the train at high speeds with the tilt feature active. So, finally, the design team chose to place the engines back-to-back in the centre of the train.{{sfn|Gilchrist|2006|p=35}} The two engines would be identical and both would carry a pantograph to pick up power, but in normal operation only the rear of the two engines would raise its pantograph, and the other engine would be fed power through a coupling along the roof. Power was converted to [[direct current]] by ASEA [[thyristor]]s, supplying four {{convert|1|MW|hp}} DC traction motors mounted in each power car. The traction motors were moved from the bogies to inside the car body, thereby reducing unsprung weight. The motors transmitted their power through internal gearboxes, cardan shafts and [[Quill drive|quill final drives]].

Other changes suggested by experience on APT-E included changes to the vertical suspension from conventional hydraulic shock absorbers to air bags, which would both improve the ride quality and have lower maintenance requirements. For service reasons, the power cars were redesigned to have their own bogies in a Bo-Bo arrangement, so they could be easily removed from the train, unlike the former articulated design that connected adjacent cars together and made it difficult to split the train apart. The passenger cars retained the articulated design, but a number of changes were made due to experience on APT-E. Finally, a system that would cause the tilt system to fail into the upright position was desired, as APT-E had failed into a tilted position on several occasions.{{sfn|Gilchrist|2006|p=35}}

As part of the same review, the team noticed that a slight reduction in maximum speed would greatly simplify a number of design points, and eliminated the need for the hydrokinetic brakes. However, the decision was made to go ahead with the original specification in order to provide the maximum possible speed. The government agreed to pay 80% of the cost of eight trains.{{sfn|Wickens|1988}}