By Daniel Garcia and James Bow.
Passenger trains require something that freight trains do not: heat and light. The coaches obtain this power off of the engine pulling the train. However, heat and power drains off energy from the engine, reducing the locomotive's tractive effort. Also, if the diesel engine were to fail, passengers would suddenly find themselves without heat or power in their cars. To compensate for this, separate Head End Power (HEP) engines are often incorporated into passenger locomotives to provide power to the passenger coaches.
GO Transit has always been an efficient, low cost operation. At first, their locomotive purchases were made with an eye to leasing or selling back locomotives to freight railroads when these locomotives were underused, or should the service ever be cut. As a result, GO ended up investing in a couple of classes of locomotives which did not feature HEP. To compensate, separate locomotives were modified with the sole purpose of providing HEP to the passenger coaches. Two series of these cars existed.
The APU Series
This class was comprised of three old F7B's, bought from Northern Pacific in the late 1970s, and shipped to General Motors Diesel Division (GMDD) in 1982. GMDD took out the traction motors and the prime mover and replaced it with a smaller 900hp and HEP generator. They were designated class CAP-60d and numbered 800-802 for the rest of their lives as GO Trains.
The demise of these units was directly tied to the fates of the GP40-2L's and the GP40u's. When those engines were sold or traded, special HEP locomotives became surplus to GO's needs. While 801 was sold to the Ontario Northland in 1994(?), the other two were scrapped in 1995.
The APCU Series
These units, much like the APU's, were used to power the coaches when a non HEP-equipped engine was used. However, unlike the APU's, these were A-cab locomotives. The cabs were retained during the rebuild so that they could be used in push-pull operations, in lieu of a cab coach. They lost their traction motors and prime mover, but got a new 900hp motor and HEP generator, as well as refurbished cabs. APCU's 900-904, which came from the Ontario Northland, were bought and modified in 1974, and were actually numbered 9858-9862 for about a year. 905-908, which were also purchased from the ONR, were shipped out in 1975 and 1976. The last two, bought from the Chicago, Milwaukee, St. Paul & Pacific, were modified in 1981 and 1982. They were numbered 910 and 911. They were designated classes CAP-60a, -60b, and -60c respectively.
Much like the APU's, the fates of these units rested on the GP40-L2's and the GP40u's. When those engines were sold/traded, GO Transit didn't need the APCU's (and APU's) anymore, so they were scrapped in 1995, save for #910. That engine was sold to Tri-Rail, presumably as a complement to GP40-L2 703.
APU Unit 800, made from a cabless F7B, is positioned between the lead locomotive and the rest of the train in order to provide Head End power. Photo donated from the Daniel Garcia collection.