The Erie triplexes were built in 1914 and 1915. They were an attempt of the locomotive designers to put as much tractive effort as possible into one locomotive. All six cylinders on these locomotives were the same size. The middle set was operated at high pressure, and exhausted to the front and rear sets, both of which operated at low pressure. There was no provision for running all cylinders at high pressure. Initially, they were numbered 2603, 2604 and 2605. They were soon renumbered to 5014 - 5016. It was said that the first triplex was capable of pulling 640 cars. However, the tractive effort that these locomotives generated was greater than the draft gear, couplers and frames of freight cars of that era could transmit. As a result, these locomotives were relegated to helper (pusher) service on the "Gulf Summit" or "Susquehanna Hill" grade near Deposit NY, and after 13 years were taken out of service in 1927 when the arrival of 2-8-4s on the Erie roster made 2-10-2s available for helper service. 5014 was scrapped in October 1929.
The sole Virginian triplex was built in 1916. It was classified XA for eXperimental, first series. Baldwin sent this locomotive to the Virginian purely on an experimental basis. Baldwin personnel stayed with the engine to try to iron out the wrinkles (without success). It was somewhat smaller than the Erie counterparts. It was also slower and ran out of steam faster than the Erie triplexes. Another difference was that it had a four-wheel truck at the rear of the tender for better tracking in reverse when drifting downgrade after pushing a train up hill. This locomotive only lasted three years before being returned in 1920 to Baldwin for rebuilding into a 2-8-8-0 and a 2-8-2 (photo). Both of these locomotives lasted until 1953.
The triplexes were used with some success on the Erie Railroad but were not without problems. They were very large locomotives and certain major repairs had to be performed in the Lehigh Valley shops at Sayre, PA in addition to Erie's own Dunmore shops. Another major problem with the triplexes was that the steam supply was inadequate for speeds past 10mph on the Erie and 5 mph on the Virginian. Part of the problem was that the tender motor unit exhausted to the air, reducing the amount of draft available to the firebox. Another problem was the "variable adhesion" of the tender motor unit. As the coal and water was consumed, the weight on drivers was reduced, thus reducing the factor of adhesion on the tender unit.
The tractive effort of these triplex locomotives was fantastic, but the boilers and fireboxes were too small to sustain any speeds above five miles per hour. The six cylinders would use all of the steam in the boiler if it was run any faster. As low speed helpers, the locomotives were a success. The Erie used them from 1914 until 1927. The Virginian found less success with theirs.