Well almost. We built a single prototype 4-unit COFC spine car at Berwick Forge & Fabricating and FreightCar America has built a number of 2-unit spine cars to carry short garbage containers with two stacked containers over the end trucks and one container at the inboard end of each unit for a total of 6 containers.
Most other articulated cars (not drawbar-connected) have an odd number of units because this results in an even number of trucks (the number of units plus one). The benefit of an even number of trucks is that you have two truck-mounted brake systems for every air brake control valve. The AAR also requires that hand brakes operate on at least half the trucks of the car so, again, having an even n umber of trucks is beneficial. This requirement was not in effect when the AT&SF 10-Pack cars were built. Even the relatively small number of spine cars with body-mounted brake rigging benefit from having one brake cylinder and slack adjuster for every pair of trucks. This is not a consideration with drawbar-connected well cars, which have self-contained brake systems with one control valve, two truck-mounted brake systems and one hand brake on each unit.
The choice between an articulated car with a large number of units versus one with fewer units is a trade-off between the greater efficiency of a car with more units over which to distribute the cost and weight of the "extra" truck, and the greater operational flexibility of a car with fewer units. TTX is no longer stretching 48' wells to 53', but continues to cut down 48' units to 40'. The cut-down 40' cars, as well as the articulated cars built new with 40' wells, are 5-unit, whereas the articulated 53' cars are built with three units. The reason for moving all well cars to 40' or 53' wells is that COFC traffic has settled into two different operations, 40' (and 20') international containers from the coastal ports and 53' domestic containers competing with 53' trailers.
Dick Dawson - retired TTC Chief Engineer and Director Freight Car Engineering at Berwick Forge & Fabricating