Can I ask, just out of interest, why do some trains tilt and others don't?
As I understand it.
Track on curves is canted (aka banked) and all trains tilt as a result of this. This both helps keep the train on the track and makes it more comfortable for passengers by turning "lateral Gs" into "vertical Gs".
However, the cant of the track is a compromise. Unlike a roller coaster, which always passes any track section at more or less the same speed, trains can pass a track section at any speed from zero to the maximum line speed. A track that is ideally banked for a given speed will be over-banked for lower speeds and under-banked for higher speeds.
"tilting trains" add an active mechanism to the train that tilts the bodies of the carriages. This, in principle at least, allows higher speeds on the same line while maintaining acceptable comfort.
In practice though, tilting trains have proven a bit of a white elephant. They add significantly to the maintenance cost and reduce the bodyshell size that will fit within the loading gauge making the interior more cramped while the gains in practice have been questionable. British rail experimented with tilting trains, building "experimental" and "prototype" versions of the APT, but they never built a production model. After privitisation Virgin Trains introduced the Penolinos (class 390) and Super-Voyagers (class 221) with tilt, however the 140 mph top speed of the pendolino was never used in service.
After virgin lost the crosscountry franchise, the new Crosscountry operator disabled the tilt functionality on their trains, so the only remaining tilting trains in the UK are the avanti pendolinos and super-voyagers. Avanti's super-voyagers are soon to be replaced by non-tilting AT-300 variants (classes 805 and 807). The new rolling stock for high speed 2 will also be non-tilt.
It remains to be seen what the future holds for Avanti's super-voyagers and pendolinos.I think there is a good chance that the super-voyagers will end up cascaded to crosscountry who are likely to disable tilt on them to match their existing fleet. As for the pendolinos I expect at least some of them to remain on classic WCML services, but with the majority of their fleet, including their flagship new HS2 trains, being non-tilt will Avanti or their successor consider it worthwhile to maintain the tilt on them?
I would not be at all surprised if, in 10 years time, there are no more tilting trains in the UK.