Third Edition

pressure relief valves, regulating valves, nonreclosing
rupture disc devices/safety vents, or a combination
device (incorporating both a rupture disc/breaking pin
and a reclosing pressure relief valve).
Pressure Relief Valve (PRV) — a reclosing springloaded
device, actuated by inlet static pressure, that
relieves excess pressure and recloses after normal
conditions are restored. “Pressure relief valve” has
replaced the term “safety relief valve”.
Pressure Tank Car — a tank car with a tank test
pressure of 100 to 500 psig built to the specifications
contained in Subpart C, Part 179, Title 49CFR.
Protective Housing — on pressure and some
nonpressure tank cars, a heavy, circular steel housing that
surrounds the fittings to protect them in the event of an
accident and from unauthorized access. Nonpressure
tank cars may be equipped with light-gauge steel
protective housings (referred to as combination
housings, breadboxes, or mailboxes) that provide
weather and tamper protection. Do not confuse with an
expansion dome as found on some (obsolete)
nonpressure tank cars. The protective housings for
cryogenic liquid tank cars are boxes or cabinets on the
sides or end of the tank car.
Regulating (Regulator) Valve — a reclosing (springloaded)
pressure relief device applied to tank cars
transporting certain refrigerated liquids (e.g., carbon
dioxide and argon) to maintain internal pressure below a
certain point. If the pressure exceeds a specific point, the
valve will open, releasing vapor, which results in autorefrigeration,
lowering the product’s temperature and
pressure.
Reporting Mark and Number — the alphabetic initial
stenciled (typically 3 or 4 letters ending with an X for
nonrailroad owned cars, followed by 1 to 6 numerals;
e.g., BOEX 2017) on the sides and ends of every freight
car to identify the railroad or private car line that owns
the car. Reporting marks are assigned by the Association
of American Railroads.
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