Double-Stack Well Car

Overview

A double-stack well car is the workhorse of North American intermodal freight. Its defining feature is a depressed center well that drops the floor between the trucks almost to rail level, so one shipping container can ride low in the well and a second container can stack on top of it while the whole load still clears bridges and tunnels. Stacking two containers in the vertical space that a conventional flatcar uses for one roughly doubles capacity per unit of train length. The car is usually built as an articulated set: several Well Platform Unit well units share trucks at their joints, riding on two End Truck end trucks and four Shared Articulated Truck shared trucks, joined to the train by End Coupler couplers. A through Air Brake System brakes the whole set, Container Securement hardware locks the boxes in place, and Hand Brake hand brakes hold it when parked.

The well platform

Each Well Platform Unit is one articulated unit built around the depressed well. Two deep Side Sill side sills form the high sides of the well and carry most of the bending load, because the floor itself is dropped so low that the strength has to live in the sides rather than under the container. A row of Cross Member cross members ties the sills together and supports the Well Bottom Support saddles that the bottom container rests on. At each end a End Bulkhead closes the well and takes the buff load of the stack, and Body Bolster bolsters carry the unit weight down to the truck center plates. The geometry is the whole point: by lowering the container into the Well Bottom Support the car gains roughly the height of one container of extra clearance for the second box on top.

Articulation and shared trucks

Rather than couple separate cars, well cars are usually articulated. Adjacent Well Platform Unit units meet over a single Shared Articulated Truck and are joined by a Articulation Connector connector, a slackless Articulation Male Connector and Articulation Female Connector pair held by a Articulation Pin. The two unit ends rest in one Articulation Bolster with a single deep center bowl, so one truck carries both. Articulation removes the slack and the coupler hardware between units, which reduces weight and the in-train forces that would otherwise build up over a long stack train. Only the two ends of the whole set carry standard End Truck trucks and End Coupler couplers to join other cars.

Trucks, wheels and bearings

The End Truck is a conventional three-piece freight truck: two Truck Side Frame cast side frames, a Truck Bolster bolster with a center plate, and two Wheelset wheelsets. Each wheelset is an Axle pressed with two 36-inch Wheel 36-inch wheels on sealed Ball Bearing roller bearing adapters. A Coil Spring spring group cushions the ride, and braking is by Brake Beam beams pressing Brake Shoe composition shoes on the treads. The Shared Articulated Truck is the heavier version with a larger Coil Spring spring group and the Articulation Bolster sized to carry two units at once.

Braking and coupling

The End Coupler at each set end is a standard knuckle coupler: a Coupler Knuckle & Head head, a Coupler Yoke, and a cushioned Cushioned Draft Gear that absorbs the long-travel buff and draft loads typical of heavy intermodal trains. Braking is the Air Brake System. A ABDX Control Valve reads the Brake Pipe & Hoses train line, manages the Combined Reservoir, and feeds the Brake Cylinder cylinders at each truck, with Slack Adjuster adjusters holding shoe clearance constant as the shoes wear and a Pressure Sensor reading line pressure. Like all freight air brakes it is fail-safe: a drop in brake-pipe pressure, including a parted train, applies the brakes automatically. For parking, each Hand Brake uses a Hand Brake Wheel geared through a Helical Gear Pair to a Hand Brake Chain that pulls the rigging tight.

Container securement

A stack only works if the boxes stay put, which is the job of Container Securement. The bottom container drops onto fixed Well Corner Casting castings in the well, guided down by Stacking Guide funnels that catch its corners. The top container then locks to the bottom one through Interbox Connector interbox connectors, twistlock fittings seated in the corner pockets where the two boxes meet, held by Pin Retainer retainers. With the bottom box located by the well and the top box tied to it, the stack rides as one mass and resists the lateral and longitudinal forces of normal train handling. Different well lengths accept different container combinations, for example a 40-foot box low and two 20-foot boxes on top, which is how a single car type serves the whole range of international and domestic container sizes.