20 AAR mechanical designations with dimensions, capacities, and commodity compatibility.
The boxcar is the most versatile enclosed freight car, featuring a rectangular steel body with sliding side doors. It provides weather protection for a wide range of general merchandise and packaged goods. Modern boxcars are typically 50 to 60 feet long and ride on four-axle trucks for heavy loads. They are the workhorse of carload freight and can be found in virtually every freight classification yard.
The covered hopper is an enclosed car with a sloped interior floor that funnels bulk dry commodities to bottom discharge gates. Hatches along the roof allow top-loading of grain, plastics, or cement. Two broad classes exist: small-cube cars of roughly 3,000 to 4,000 cubic feet for dense commodities like cement and fertilizer, and large-cube cars of 4,500 to 6,000 cubic feet for light-density grain and plastics. The bottom outlet gates are pneumatically or gravity-discharged at destination.
The open hopper is a self-clearing car with sloped interior hopper bays and bottom discharge doors, but no roof enclosure. It is purpose-built for bulk commodities that do not require weather protection and are delivered by top-loading equipment such as rotary car dumpers or bottom-dump facilities. Coal hoppers traditionally carry 100-ton payloads and operate in unit trains of 100 or more cars. Aggregate hoppers are typically shorter and heavier-built to accept large rocks.
The gondola is an open-top car with a flat or slightly contoured floor and fixed or drop ends and sides. Unlike hoppers, gondolas are manually or mechanically unloaded because they have no bottom discharge. Mill gondolas serve steel mills continuously cycling between furnaces and rolling operations. Gondolas also haul scrap metal, pipe, lumber, and heavy industrial products. Modern gondolas are often 52 to 65 feet long and carry 100-ton payloads.
The tank car is a cylindrical pressure vessel mounted on a rail underframe, designed to transport liquids, liquefied gases, and slurries in bulk. Tanks may be insulated, heated, or cryogenic depending on the commodity. The DOT-111 is the most common general-purpose car, while DOT-117 (the post-2015 enhanced standard) features a thicker steel shell, full head shields, and thermal protection. Pressure cars (DOT-105, DOT-112) handle liquefied gases such as propane, chlorine, and vinyl chloride at higher containment pressures.
The flatcar is the simplest rail freight car — a level steel deck with no sides, ends, or roof. Its open configuration makes it suitable for over-dimensional or awkward loads that cannot fit inside an enclosed car. Standard flatcars use stake pockets, chains, and tie-down rings to secure loads. They are used for machinery, military equipment, large rolls of newsprint, and project cargo. Modern flatcars range from 48 to 89 feet in length.
The well car — also called a stack car — has a depressed center section (the well) that sits between the trucks, allowing ISO intermodal containers to ride lower than on a standard flatcar. This extra vertical clearance permits double-stacking two 9'6" tall containers within standard railroad clearance envelopes. Double-stack well cars dramatically improve train density, reducing per-container transportation cost by as much as 40 percent compared to single-stack service. They are the backbone of transcontinental intermodal service.
The autorack is a multi-deck enclosed car designed exclusively to transport finished automobiles and light trucks from assembly plants to regional distribution centers. The enclosed steel rack structure protects vehicles from weather, vandalism, and road debris during transit. Modern enclosed autoracks replaced the older open-rack designs in the 1980s after widespread vandalism problems. Tri-level racks carry compact and mid-size cars; bi-level racks carry full-size trucks, SUVs, and vans with greater height requirements.
The centerbeam flatcar has a full-height longitudinal center partition running the length of the car, with load secured to both sides of the beam using straps and load bars. This bilateral loading configuration doubles the load density compared to a standard flatcar for certain commodities. Centerbeams are primarily used in lumber and building products service in Western North America. The center spine provides structural rigidity and a tying surface for bundled lumber, wallboard, and particleboard.
The refrigerator car, commonly called a reefer, provides active temperature control for perishable commodities during transit. Mechanical reefers use diesel-powered refrigeration units mounted in an end compartment to circulate cooled air throughout the load chamber. Cryogenic reefers use liquid nitrogen or liquid carbon dioxide instead of a mechanical compressor for longer retention times or ultra-cold requirements. Modern mechanical reefers maintain temperatures from -20 degrees Fahrenheit to +70 degrees Fahrenheit with precision control.
The coil car is a specialized gondola or flatcar fitted with permanent saddle-shaped cradles to transport steel coils in an upright orientation. Steel coils are extremely dense and concentrated-load items that would damage a flat car deck or roll uncontrollably inside a standard gondola. Coil cars spread the load across multiple cradle supports and keep each coil from contacting adjacent coils. The cars cycle between flat-rolled steel mills and stamping plants, service centers, and pipe mills.
ISO intermodal containers are standardized steel boxes that transfer between ships, railcars, and trucks using the same standardized corner fittings without rehandling the cargo inside. They are not rail cars themselves but are the load units carried by well cars, flatcars, and spine cars. The ISO standard governs exterior dimensions, corner casting locations, and structural strength requirements. North American domestic containers (48 ft and 53 ft) are wider and taller than international ISO containers but use the same corner fitting concept.
The spine car is a skeletal intermodal car consisting of a central steel spine or chassis with articulated or independent platforms, designed to carry containers or trailers with minimal dead weight. Because it lacks the heavy well structure of a well car, it is lighter and less expensive, but it cannot double-stack. Spine cars are commonly used for trailer-on-flatcar (TOFC) service and for containers where clearance does not permit double-stack operations. They are often built as multi-unit articulated sets to share trucks and reduce tare weight.
The depressed center flatcar has end decks at normal rail-car height with a center section significantly lower, typically 12 to 18 inches below the end sections. This configuration reduces the overall height of tall loads, allowing them to pass under bridges, overhead catenary, and tunnel portals that would otherwise be clearance-restrictive. Depressed center flatcars are used for electrical transformers, large industrial machines, and other tall heavy loads. They are often paired with multi-axle bogies to spread the concentrated weight over more rail.
The Schnabel car is the most specialized rail freight equipment in North America, designed to carry a single extremely heavy and oversized load by actually becoming part of the car structure. Two end-frame assemblies with multiple axle trucks attach directly to fittings on the cargo itself, so the cargo (typically a large transformer or reactor) serves as the central structural member of the car. Schnabel cars can carry loads exceeding one million pounds and are custom-designed or purpose-matched to specific loads. Movement requires careful route clearance surveys, utility wire lifts, and slow-speed operation.
The pressure differential car, commercially known by the Airslide trademark, is a covered hopper designed with a porous membrane floor through which compressed air is pumped to fluidize fine dry bulk commodities. The air-fluidized product flows like a liquid to pneumatic conveying lines connected at discharge outlets. This design eliminates bridging and hangup problems common with fine powders in standard gravity hoppers. Airslide cars are commonly used for Portland cement, fly ash, flour, and other cohesive powders.
The bulkhead flatcar is a standard flatcar fitted with large fixed vertical steel bulkheads at each end, typically 4 to 8 feet tall, to contain and restrain loads that would otherwise shift longitudinally during braking and acceleration. The bulkheads protect the cargo and prevent it from sliding off the ends of the car. Bulkhead flatcars are the dominant equipment for lumber, pipe, and steel bar in unit train and manifest service. They are often used in conjunction with centerbeam flatcars on the same trains serving the building products industry.
The log car is a skeletal flatcar with steel stanchions or bunks at intervals along a central spine, designed to cradle and secure full-length logs during transit from forest to sawmill. The open framework minimizes tare weight while the fixed or adjustable bunks support the logs and prevent them from rolling. Log cars often operate in dedicated shuttle service between timber loading facilities and sawmills, sometimes loaded by log-stacker cranes directly from logging trucks. They are predominantly used in the Pacific Northwest and Southeast.
The rotary dump gondola is a heavy-duty gondola or open-top car specifically equipped with rotary-end couplers on one or both ends, permitting it to be rotated 360 degrees in a rotary car dumper while remaining coupled to adjacent cars in the train. This allows extremely rapid unloading — a full 100-ton car can be emptied in under two minutes — without uncoupling or spotting individual cars. Rotary dump gondolas are the dominant equipment for coal unit trains serving power plants and export terminals with rotary dumper installations.
The multi-level auto carrier — commonly called an autorack — is the enclosed multi-deck rail car used for finished vehicle distribution. It consists of a standard flatcar or well flatcar underframe surmounted by a bolted or welded steel rack structure with two or three decks, full-height corrugated steel side panels, and hinged end doors. Modern enclosed designs replaced open racks following extensive vandalism losses in the 1970s and 1980s. Vehicle loading is via ramps at end doors, and cars travel in dedicated unit trains between assembly plants and vehicle distribution centers.