Common contraction of the French mouthful Union Internationale des Chemins de Fer -- International Association of Railroads, more or less. It was founded in 1922.
The stated role of the UIC is to promote cooperation between railway systems at a world level and to promote and develop international rail transport.
International standards for rail equipment are one of the most important achievements of the UIC. This enables interoperability across national boundaries, allowing locomotives and particularly freight and passenger cars to cross borders and be able to operate in trains in other countries. UIC standards are wide ranging and include wheel and track standards, coupling standards, numbering standards for cars, braking standards, and many others.
The UIC also funds, encourages and coordinates research into new railway technologies, both by doing research itself and helping to coordinate research done by individual railroad companies.
The third function of the UIC is the promotion of rail transport. It does this through political lobbying, through contacts with the press, through exhibitions, conferences and seminars, etc.
Like many international organizations, the UIC is pretty much ignored in the United States. American railroads do not follow UIC standards in the slightest; instead, they follow standards laid down by the AAR, the Association of American Railroads, which do not correspond with UIC standards.