The Black Diamond inspection engine, though considered by some to be a bit of an oddity, appears to have been commonly used by railroads in the latter half of the 19th century and the first two decades (or more) of the 20th century. Many of them are odd looking beasts, often looking like a cross between a carriage and a steam locomotive.

The other "Black Diamond". The Reading Company owned at least 13 of these tiny inspection locomotives.

Nantes - Tramway - Ligne 3 - Orvault by IngolfBLN, via Flickr

Nantes - Tramway - Ligne 3 (Orvault) One of the reasons I like the tram in Edinburgh, thinking this could be the look of the tram line someday.