Monorails are somewhat cheaper than elevated trains (if you don't have extreme disabled-evacuation laws) but more expensive than a conventional train at ground level, and junctions are a nightmare. So they only work where you have a single isolated line and would need to elevate most of the track anyway (and suspended rather than straddle-beam improves cornering performance but at a cost, so is only worth it if your route has many sharp corners as well).
For Wuppertal, where the town is pretty linear along a river valley, it works. (Even then, a straddle-beam monorail would probably be more cost-efficient if you were starting from scratch). For most places it doesn't.
Monorail needs bigger tunnels than conventional rail so if you need underground sections - which most systems need - they are more expensive.
even if you are only need a single isolated elevated line though monorail still loses just because there is no standard: when you need spare parts in 20 years it is questinable if you can get them.
> Monorail needs bigger tunnels than conventional rail so if you need underground sections - which most systems need - they are more expensive.
It's rare to need much in the way of tunnels on an elevated line, by its very nature. And since the beam is narrower than the carriages, it doesn't actually increase the tunnel diameter (for a round cross section) that much.
> even if you are only need a single isolated elevated line though monorail still loses just because there is no standard: when you need spare parts in 20 years it is questinable if you can get them.
These days there are systems that have been built for decades by big name manufacturers, often the same manufacturers that make trains. Hitachi or Alstom-nee-Bombardier aren't going to disappear and leave their clients high and dry, if only because it would be bad for their broader rail businesses to do so.
I'm no fan of monorails - quite the opposite - but there are cases where they work.
Those are still single vendor systems so if your vendor disappears - as has happened - you are stuck. You also don't get competition for spare parts so who knows if the price will be reasonable in the future.
Isn't it pretty similar for any regular railway? Sure the physical track might be more standard, but that's not where the expense comes in, it's the signalling system, the trains, and especially the interface between the two - and that's all going to be the same either way.
It's arguable for modern rapid transit signalling systems, because those tend to be a bit more bespoke than mainline railway systems and you might not be able to switch to a different supplier without swapping out all the interlockings, and the trackside and train-borne equipment in one go, but for the trains I don't follow your point – switching manufacturers when ordering the next generation of trains is par for the course, and having the signalling system of your choice plugged into the train of your choice is standard procedure even on rapid transit and light rail systems.
Bangkok changed the plans of its two most recent transit lines (Yellow and Pink) from standard rail to monorail for the cost savings - since they're completely elevated. I guess they solved the evacuation issue by making the space between the two rails a solid platform.
For Wuppertal, where the town is pretty linear along a river valley, it works. (Even then, a straddle-beam monorail would probably be more cost-efficient if you were starting from scratch). For most places it doesn't.