[highcountess]

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.

[bombcar]

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.

[cesarb]

> but you could (at much increased weight) put wheels on both the top and bottom of the carriage and have a transition point.

A fictional example of this is the monorail on the Half-Life games, which transitions between straddle-beam and suspended.