In Ouray, Colorado I came across this narrow-gauge rail bike outside a defunct silver mine. Its not an eco-friendly relic of old mining days but was built recently by a crafty local citizen. There's lots of storage space and even racks for hauling mining supplies on this rail bike.
There wasn't much track to try it out on or time before someone chewed me out, but it rolled well and I imagine you could build up a lot of inertia with those solid metal wheels. There are no brakes, only the fixed-gear pedals to slow you down.
Down by the historical museum there's another example of this person's work. Of course there is no need for steering or handlebars, but its a comfortable place to rest your hands while riding.