Hawaii is trying to add a light rail to provide fast transit service across the island. Like all major transit projects, it is a complicated process. The proposed corridor is about 20 miles long, far too long for streetcar or a single bus route. Light rail has been chosen as the mode. But beach-front property in Hawaii is not exactly cheap. Development is pretty dense.
In similar situations, subway is typically built. The Hawaiian islands are volcanic outcroppings; adding so much as a sewer line requires blasting with dynamite. I imagine it would be possible to bore 20 miles of tunnel using special machinery, but the NYC Second Avenue subway ran over $2 billion/mile, so it is an unaffordable option.
The next best option is re-using existing Right of Way (ROW). But a two-track wide light rail corridor would be 25' wide, either forcing the LRT to share ROW with automobiles, or eliminating automobiles from the road. The former would destroy any hope of speed or reliability for the LRT; might as well use a bus. I doubt the latter is politically feasible.
Thus, the current initiative is to elevate the light rail. As you would guess, there has been some considerable outcry against this--it would be similar in appearance to a freeway overpass. Needless to say, there was some outcry, notably from the AIA, (American Institute of Architects), on the aesthetics of an elevated railway.
Wikipedia offers a potential alternative:
Wuppertaler is the strange example of a successful mono-rail. It is a suspended mono-rail, using a single overhead rail. This also allows it to minimize the visual impact of overhead wires. Rather than the gargantuan concrete pillars that so blight Seattle, it has much nicer looking iron trusses for support. I imagine it must also be lower cost to maintain--the UTA Trax tramway requires regular sweeping of debris and trash to remain safe. The worst an overhead rail would need to worry about would be pigeons.
A suspended monorail is a viable option for the Honolulu corridor. One of the main problems with a monorail is the astonishing difficulty in changing tracks--it is so time consuming as to be impossible. This makes extending the monorail very difficult, especially if one of the original ends is a dead-end. Honolulu is a single linear corridor with minimal potential for perpendicular routes.
Monorail is typically cheaper than light rail. One of the substantial costs for street-running light rail is the need to move substantial utilities, an extra- expensive prospect for Honolulu. While the support trusses would require some earthwork, they are more numerous than concrete support pillars, allowing the weight to be distributed more evenly, reducing the cost of the associated earthwork.