Editing Advanced Passenger Train (section)

===POP===
Although construction of the train was relatively straightforward, a number of more serious problems appeared in the power and control systems. Thus the decision was made to build two additional power cars as unfinished frameworks with no power. These cars would instead be hauled by conventional locomotives to provide data on the tilting and braking systems as well as the dynamics of the vehicles. A contract for the additional two cars was sent out on 14 April 1970, and ran for the first time in September 1971. The name "POP" was assigned, an acronym for "power-zero-power", indicating the two power car layout with no passenger cars in the middle.{{sfn|Gilchrist|2006|p=33}}

The selection of a space frame design for the power cars turned out to be fortunate, as during construction the engineers concluded that the packaging of the various elements within the car would render it dynamically unstable. They needed more room to spread the parts out, so the decision was made to roughly double the length of the power cars. This turned out to be easy to do; the frameworks already under construction at Metro-Cammell simply had additional sections of steel tube inserted and construction was barely affected.{{sfn|Gilchrist|2006|p=33}}

The POP cars were skinless, topped with a space frame holding ballast to simulate the various parts of the prospective design. The "POP" acronym was soon rendered inaccurate when a passenger car was added to make a three-car train, at which time the power cars were also given bodies. The POP underwent a number of changes, notably trialling different bogie designs, over its lifetime.{{sfn|Wickens|2002}}