Bukkit: 1.6.2 (requires TrainCarts v1.54)
The Hang Rail add-on was a TrainCarts request that I liked as well. However, it was a bit too 'different' to just put it in Train Carts (causing everyone that used iron bars as visual styling to get bugs). For that reason, you can install this add-on next to TrainCarts to add minecarts suspended on iron bars.
Other than that the rail is now not below the minecart but above it, there is no change in minecart movement physics. You can make turns, go up and down and can even make X-crossings and T-splits. Everything blends in with other rail types by utilizing the (new) TrainCarts rail API.
Install TrainCarts and then add the jar file of this add-on to the plugins folder as well. It will automatically register the new available rail type.
By placing iron bars in a horizontal line you can create tracks. To get a minecart on these tracks, place a normal track leading to it 2 blocks below it. When the minecart leaves the tracks it goes onto the hang rail and stays there. There are no commands or permissions you have to set up: just place the rails and you're done.
TrainCarts signs can be placed on top of the rail, that is, place a block (such as stone, grass, wood, etc.) on top of the rail and attach signs to it. All signs, including stations, spawners, etc, function. Path finding also supports hangrails.
How to show that suspension block above?
You can change the block displayed in the Minecart to be this block. In this case, use the two commands:
/train setblock cobblewall /train blockoffset 48
Similarly, with a blockchanger sign:
[train] blockchanger 48 cobblewall
Stations and sign usage
Horizontal line with some curves
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