Editing Advanced Passenger Train (section)

===Cant and tilting trains===
[[File:Railway superelevation at Dunbar, July 2012.jpg|thumb|right|Superelevation was applied to portions of the BR network, although the angle was limited. Here, an [[InterCity 225]], based on APT technology, rounds a canted turn on the [[East Coast Main Line]].]]

During this period, BR's Passenger Business division produced a report suggesting rail could compete with road and air, but only if the trains ran faster. Studying the increase in ridership due to the introduction of the [[British Rail Class 55]] "Deltic" engines on the [[East Coast Main Line]], and the effects of electrification on the WCML which improved journey times 20 to 30%, they concluded that every {{convert|1|mph}} increase in speed would result in a 1% increase in passengers. This basic rule was apparently proven in Japan, when the Tokyo-Osaka [[Shinkansen]] line was operating from 1964 to huge success.{{sfn|Wickens|1988}}

The Shinkansen provided a smooth ride at speeds as high as {{convert|125|mph|0|abbr=on}} by laying new lines dedicated to high speed travel. BR's most used route, the WCML, had in the order of 6 million passengers a year between [[London]] and [[Manchester]],{{Dubious|date=April 2018}} a far cry from the Tokyo-Osaka's 120 million. Funding for a new line for high speed use was highly unlikely given these passenger levels.{{sfn|Wickens|1988}} This presented a problem for any sort of high-speed operation on the route because the existing line contained many turns and curves, and rounding these at high speed would cause lateral forces that would make walking difficult, and throw items off tables onto the floor.

The traditional solution to this problem is to tilt the rails into the turns, an effect known as ''[[Cant (road/rail)|superelevation]]'' or ''cant''. This has the effect of making the lateral forces more inline with the floor, reducing sideways forces. Because larger amounts of cant are more difficult to construct and maintain, and also because of the need to account for slower-moving traffic or the possibility of a train coming to a stand within the curve (both of which cases would consequently experience a force to the inside of the curve, a condition known as ''cant excess''), long experience had shown that the maximum amount of cant that could be applied to lines with mixed traffic was 6.5 degrees.<ref>{{cite book |last1=Weigend |first1=Manfred |editor1-last=Fendrich |editor1-first=Lothar |editor2-last=Fengler |editor2-first=Wolfgang |title=Handbuch Eisenbahninfrastruktur |date=2013 |publisher=Springer Vieweg |location=Berlin Heidelberg |isbn=978-3-642-30021-9 |page=613 |edition=2nd |language=de |chapter=Kapitel 12 – Trassierung und Gleisplangestaltung}}</ref>

Given the curve radii typically encountered on the WCML, this meant that even with the maximum permissible amount of cant applied, speeds couldn't be increased much above the {{convert|100|mph|0|abbr=on}} range without once again experiencing excessive lateral forces. As the initial factor limiting speeds is ''not'' safety against derailing or overturning, but rather only passenger comfort, the solution to increasing speeds further is therefore having the train car bodies tilt as well – while this doesn't influence the forces acting at the wheel-rail level, it keeps the lateral forces experienced inside the passenger compartment at a comfortable level even at further increased speeds.

[[Talgo]] introduced the first practical design for a tilting carriage in the late 1950s. This consisted of a single bogie placed between the train cars with the car bodies suspended from an A-frame centered on the bogie with a pivot near the top. When the train rounded a bend, the centrifugal forces caused the car body to swing out like a pendulum, reaching the proper tilt angle naturally. However, this system had a distinct delay between entering the curve and the body swinging out, and then swung past this angle and then oscillated briefly until settling at the right angle. When traversing a series of curves, like in a switchyard, it tended to swing about alarmingly. Although a number of semi-experimental designs of the 1970s made use of it, like the [[UAC TurboTrain]], the concept was not widely used.{{sfn|Wickens|1988}}