U-FACTOR
The total thermal conductance of a particular insulated tank. It is a measure of insulating efficiency and varies with the type and thickness of insulation used.

U.S. CODE OF FEDERAL REGULATIONS
See Code of Federal Regulations.

ULTRASONIC INSPECTION
A nondestructive test method that transmits ultrasonic energy through the test material to detect internal and surface flaws. Used also for shell plate thickness measurements.

UMLER
Acronym for Universal Machine Language Equipment Register. A computerized file maintained by the Operating Transportation Division of the AAR. UMLER contains specific details on internal and external dimensions of equipment as well as special equipment and the general information shown in The Official Railway Equipment Register.

UNACCEPTABLE WELDING DEFECT
Any weld defect that exceeds the standards contained in Appendix W of the AAR Specifications for Tank Cars, M-1002.

UNCOUPLING LEVER
The manually operated mechanical device mounted at each end of a railcar used to unlock and open the coupler.

UNDERCUT
A groove inadvertently melted into the base metal adjacent to the toe or root of a weld and depending upon size, could be classified as an unacceptable welding defect.

UNDERFRAME
The term used to refer to the entire structural framework of the car below the floor, including the center sill, side and end sills, bolsters, cross members, stringers and other attached components.

UNFAIR USAGE
Damage to a car caused by improper operation.

UNIT TRAIN
A train of like cars used in dedicated service.

UNLIMITED INTERCHANGE
Refers to a railcar whose physical dimensions are such, as to allow it to be operated on all North American railroads without restriction. This does not include private sidings.

URETHANE
A term used to refer to certain highly durable paints or to foam insulation that contain urethane resins.