The Southern Pacific 72-foot suburban steel coach was first built in 1923 by the Pullman Company. The Southern Pacific Railroad ordered them specifically for San Francisco Bay Area commuter service to replace their aging fleet of wooden passenger cars. The first order was for 60 coaches, seating 96 passengers each. They were an instant hit with commuters, so another 10 were ordered in 1924. Five more were ordered from the Standard Steel Car Company in 1927. These coaches were used in commuter service until 1985.

Although many people now refer to the coaches as Harrimans, most people used call them Subs. The term Harriman refers to railroad equipment built in the 1900's and 1910's to a set of common standards that were used by the Harriman controlled Southern Pacific and Union Pacific. A quick way to identify a Harriman car is to look at the letterboard. On a true Harriman coach, the letterboard continues over the doors. Although Subs are quite similar to Harriman coaches, they are technically not true Harrimans because the letterboard stops at the doors.

The coaches are 72'0" long over the end sills, 9'9-7/8" wide, and 14'0" from rail to top of roof. They are 80'8-7/8" long from coupler to coupler. Each coach weighs 119,900 lbs. without passengers. The coaches are constructed on a steel frame with steel channel and plate used throughout. The coach floor is poured concrete, which provides a smoother and quieter ride. Interior lighting is powered by an electric generator or by batteries. A generator is connected by belts to one axle, and it charges the batteries while the coach is moving. Each coach has overhead fans for ventilation, and is heated by steam vapor supplied by the locomotive.

Even though the Subs were all built to the same specifications, there are minor differences between the orders. The first two orders were built by Pullman, and were designated as classes 72-IC-1 and 72-IC-2. Class 72-IC-2 had slightly different window sills, but was otherwise an exact duplicate of the 72-IC-1 class. The third order was built by Standard Steel Car Company (before Standard Steel was part of Pullman), and it is class 72-IC-3. Nicknamed "Cream-Puffs", the cars in this class had a smoother ride. They used cast steel trucks with integral journal pedestals cast as part of the truck frame, instead of the older style with bolted pedestals.