Not sure if this one's been mentioned or not so apologies if it has, but
River King Mine Train at Six Flags St. Louis.
So it's an Arrow Mine Train, with their weird hybrid support system, but that's nothing that special really. There's 14 others operating around the world at various Six Flags and Cedar Fairs attractions, and PortAventura to name a few. But it's history tells an interesting tale.
It first opened back in 1971 as River King Mine Train, where it operated for 12 years to 1983, with the only notable change being it gaining the word Runaway to its title a year after opening.
However, for 1984 the ride opened renamed as Rail Blazer, complete with, wait for it,
standup trains! Yep, standup trains on a mine train coaster! It's age means it was amongst the first standup coasters, but it was only a short-lived stint. The trains lasted one season before they were removed due to someone dying on it, after which the ride reverted back to its original title and trains, which it has operated with ever since.
There are several theories about how the woman died on it, but the official stance was apparently that she fainted and managed to slide out of the train, which looking at the (lack of) restraint and train design seems incredibly plausible. There doesn't appear to be much to stop riders ducking their head out from under the OTSR, and there doesn't look to be any form of saddle to hold the body up with.
Looking at a POV it looks pretty dull as even a mine train, so god knows what it was like as a standup!
http://www.youtube.com/watch?v=p-4blDzPy1s#