The M'O is one of the more interesting museums in Paris, which may explain why we spend a minimum of one day there every time we find ourselves in Paris. Once upon a time (Il y a un fois...) it was the Palais d'Orsay before it became the Gare d'Orsay, a railway station, and served most of southwest France from about 1900. In the early 20th century trains were electrified and grew longer, but Orsay's platforms didn't and its function was eventually taken over by other stations, notably Gare Montparnasse, until it was abandoned after the second World War.
Since the structure was sound there was no reason to tear it down as we might do here; the French have a great respect for beautiful things, and the Gare d'Orsay was too beautiful to lose. They did the logical thing and converted it to another use (1986), and today it is a museum that preserves much of the grandeur of the railway stations of La Belle Époque. In particular, the huge clocks that once told travellers the time are still operating at M'O and can be seen on the side of the building which faces the Seine.
More than anything else, two things draw us back to M'O: their collection of Impressionists, especially Claude Monet, and the sculptures of Charles Cordiere.