URR or Urr may refer to: Urea reduction ratio, used to quantify dialysis treatment adequacy Urr Water, a river in Scotland Haugh of Urr, a village by the Urr Water, in the Urr parish Motte of Urr, a "motte-and-bailey" castle near the Haugh of Urr, beside the Urr Water Union Railroad is a name, or part of a name, that has been used by many companies, mainly in the US Union Railroad (Pennsylvania) is possibly the only current user of that name Underground Railroad, an informal escape network used by 19th-century black slaves in the US Ultima Ratio Regum, a historical roguelike game