This does seem like a superior way to build elevated rail. Less noise in particular, as turning doesn't induce slippage like on a normal train. Wonder why it's so rare.
Lots of unnecessary complexity. In this case it makes sense because the majority of the line is directly over a river due to space constraints, but it's a lot simpler to build a concrete viaduct and run normal trains ovwr it. This also allows the train to transition to run underground or at-grade.
Not an engineer, but just looking at the photos, this takes an enormous amount of steel. While most elevated rail is just a concrete bridge with a small amount of structural steel.
Most rails lines continue far enough to leave dense urban areas where this makes sense so they have to transition between elevated and ground level tracks which this can't do.
Having lived there for several years I remember these to be quite loud. Maybe because the way they hang allows them to sway left to right a bit, and causes the metal wheels to make contact with the track at various angles. Quiet they were not, but fun to ride for sure, and a lot more punctual than busses.
Turning also doesn't involve slippage in trains. Many locomotives already have independently turning wheels, and the solid axles of train carts can also turn just fine (as explained in https://www.youtube.com/watch?v=f_-UJNETSlg for example)
The solid axle turning without slippage from that video works for regular inter-city rail. I think it doesn't work for rails with the tighter turning radii often required of inner city trams and similar rail transport.
I think the lock-in is the biggest issue. If you have a hanging rail system, you can't just transition off the hanging rail to bottom rail when no longer needed like you could with elevated bottom rail.
This one has the advantage of always being over the river, but you could (at much increased weight) put wheels on both the top and bottom of the carriage and have a transition point.
Expensive and impractical, but could be fun at a theme park.
