For the South Side Rapid Transit in Chicago, it was required that the trains with five four-axle passenger cars should achieve an average speed of 15 mph. This was higher than, for example, on the New York Elevated and required extra powerful Forney locomotives. Thus, in 1892 Baldwin delivered a total of 45 of these locomotives, which were primarily optimized in terms of power.

Unusual for a locomotive of this size was that they were four-cylinder Vauclain compounds. All cylinders could be supplied with live steam for starting, which ensured high starting tractive effort and rapid acceleration. Thanks to a large firebox, it was possible to use slow-burning anthracite, which reduced smoke emissions and simplified the crew's work.

Another locomotive with the number 46 was also built as a compound, but with only two larger cylinders. Since this did not prove itself as a compound, it was simpled and used in switching service.

The power of the locomotives was fully utilized with a timetable that had two-minute intervals at rush hour. The network was electrified as early as 1898, so the steam locomotives were sold. Many locomotives ended up in industry or logging service, where their flexibility was an advantage.