PACKING
A general term that refers to the sealing material around a valve stem.

PAD
On a tank, a plate contoured and welded directly to the tank surface upon which another structural apparatus is attached. Also, a pressurized inert gas, such as nitrogen, in a tank.

PASSIVATION
A nitric acid treatment commonly applied to the interior of stainless steel tanks to enhance corrosion resistance, generally preceded by a cleaning treatment.

PATCH PLATE
A steel plate applied as an overlay in a corroded area.

PAYLOAD CAPACITY
The maximum number of gallons that can be legally loaded into a tank car.

PEDESTAL
The portion of the truck side frame that takes the vertical load from the wheel bearings.

PER DIEM
A daily rate on freight cars.

PHENOLIC COATING (HIGH BAKE)
A thermosetting resin or plastic primarily used for interior coating of tank cars.

PIH
Poison Inhalation Hazard.

PIPE BOLSTER
A term used to describe the Union Tank Car standard body bolster design, whose outside member consists of a section of steel pipe.

PIPE BRACKET
One of the three parts of an air brake control valve.

PIPE CLAMP
A clamp for holding brake air pipe in place.

PIPE COUPLING
A one piece threaded fitting used to connect two lengths of pipe.

PIPE UNION
A threaded fitting assembly used to connect two lengths of pipe without having to rotate the pipes.

PISTON TRAVEL
The amount of linear movement of the brake cylinder piston rod when the car air brakes are applied.

PITTING CORROSION
Localized corrosion defined in Appendix "R" of the AAR Design Manuals.

PLACARD CARD HOLDER
A steel frame designed to hold and display cards describing the nature of a dangerous commodities being transported in a railway tank car.

PLATE B
The AAR Clearance diagram for unlimited interchange. See "Clearance Diagram."

PLATE C
An AAR clearance diagram for limited interchange. See "Clearance Diagram."

PLATE F
An AAR clearance diagram for limited interchange. See "Clearance Diagram."

PLATFORM GRATING
A flat surface formed by welding thin strips on edge in a crisscross grid pattern or a formed surface which permits drainage while forming a solid support for the foot.

PLUG VALVE
A valve whose sealing member (plug) is raised and lowered against a seat to open or close the valve. May be handwheel or lever operated.

PLUG VALVE (CYLINDRICAL)
A type of valve that is similar to a ball valve except that a cylinder with a hole through it is used in place of a ball. This type of valve is available with a full Teflon liner inside the valve and is occasionally used on tank cars for loading and unloading highly corrosive products.

PLUG VALVE (RISING STEM)
A valve whose sealing member (plug) is raised and lowered against a seat to open or close the valve. May be hand wheel or lever operated.

PLUG WELD
A circular weld made through a hole in one member of a lap or tee joint joining that member to the other.

POST WELD HEAT TREATMENT
Generally refers to the heating (1200 degrees F) and subsequent cooling of all completed tanks in a large oven for the purpose of relieving weld induced stresses.

POTTING COMPOUND
A substance (commonly silicone rubber) used to fill the dead space formed between some bottom outlet valves and the saddles to which they are mounted. This eliminates commodity collecting in this space and possibly deteriorating and contaminating subsequent loads.

PRESENT VALUE
A mathematical calculation of the current value of future revenues.

PRESSURE
A unit force generally measured in pounds per square inch (or kilopascals, KPa) created by the action of a compressed gas or fluid in a confined space such as a tank car tank.

PRESSURE CAR
A tank car built to DOT specs 105, 109, 112 and 114. Reference 49CFR Part 179 Subparts A, B, and C. Pressure cars have only one top nozzle which serves as a manway and upon which the fittings are mounted.

PRESSURE HEAD
The valve configuration on the top of a pressure tank car.

PRESSURE REGULATOR
An adjustable device generally used to reduce the high gas or fluid pressure from a primary source to any one of a number of desired lower pressures. Not to be confused with the pressure regulator valves used on Liquid CO2 tank cars which are non-flow rated valves set to automatically vent the normal pressure buildup, that occurs in CO2 cars, as this buildup reaches a predetermined level.

PRIME
The application of an initial coat of paint to a car, when required, to facilitate the adhesion of a subsequent top or finish coat of paint.

PRIVATE CAR
A car not owned by a railroad; reporting mark ends in "X."

PRIVATE CARRIER
A carrier that provides a service for the movement of goods owned by the vehicle operator.

PRIVATE SIDING
A side track owned or leased by an individual or a firm.

PROGRAM DISMANTLER
Those railcars in the fleet determined to be obsolete. These cars are sent to a scrap metal dealer upon lease expiration.

PROTECTIVE HOUSING
Generally refers to the heavy steel housing surrounding the fittings mounted on pressure car manways.

PULL IRON
See "Roping Staple."

PURLINE
A longitudinal roof support member sometimes used on covered hopper cars and other "House Type" railcars.

PUSH ROD
The steel rod mounted within the hollow brake cylinder piston rod that transmits the brake cylinder force to the brake rigging.

PV CASH FLOW
See "Present Value," used to calculate the return on investment of a car.

PWHT
Acronym for post weld heat treatment. See "Post Weld Heat Treatment."