[AngusH]

Are you thinking of the Shanghai maglev?

It is pretty impressive and they're looking at building an extension. however oddly its operation speed has been significantly reduced from 268mph to 186mph.

https://en.wikipedia.org/wiki/Shanghai_maglev_train

The basic problem is that Maglev is on average more expensive to build and operate and cannot (unlike ordinary high speed rail trains) ever operate on ordinary (non high speed) rail track.

It's also still pretty experimental, the Shanghai one is operational and high speed, all other operational systems are either low speed or experimental test beds.

https://en.wikipedia.org/wiki/Maglev

Regular high speed rail is a fairly routine thing and all the parts are well understood.