with a fuel tank. Subsequent units were built with a tank. Beginning with the EMD F40PH-2, introduced in 1985, the prime mover developed . Many of the original F40PHs were updated to match that output. The locomotives were long. A standard F40PH weighs . For passenger service the F40PH has another electrical alternator, the head-end generator. The HEP unit generates three-phase AC power at 480 V (500 kW on the first order, 800 kW on later units) for lighting, heating and air-conditioning the train. Originally, F40PHs powered the HEP alternator from the prime mover. As a result of that, the train had to be at a constant 60Hz frequency, and the prime mover had to turn at a constant 893 RPM while supplying head end power (even standing still, with the throttle in idle). Power to the traction motors was controlled by varying the field excitation of the main (traction) generator. On some later versions of the F40PH (and on many rebuilt F40s), a second small auxiliary diesel engine at the rear of the locomotive powers the HEP alternator. In these engines, the prime-mover speed varies in the usual way. They can be identified by the diesel exhaust at the rear of the locomotive and by their quiet idle. Remaining F40s, with the constant-RPM prime movers, are noticeably louder. In the initial design the battery box and air reservoirs were located forward of the fuel tank. In locomotives manufactured after 1977 these were relocated behind the tank. The battery box returned to its original location in the F40PH-2. The F40PH-2s delivered to Caltrain incorporated Southern Pacific Railroad-style headlights. The designation "F40PH" stood for the following: "F" for the full-width cowl carbody, "40" as the locomotive is part of EMD's 40-series (based on the GP40-2 freight locomotive), "P" for passenger service, and "H" for head-end power. History Amtrak ordered its first 30 F40PHs on May 8, 1975; the first of the new locomotives entered service on April 9 the following year. Amtrak intended the locomotives for short routes such as the San Diegan in California and Northeast Corridor services in the then non-electrified portion between New Haven, Connecticut and Boston, Massachusetts. The long-distance routes were protected by the then-new EMD SDP40F, described by J. David Ingles in late 1975 as the "stars of Amtrak's long-distance trains". Two events led to a major change in thinking within Amtrak. The first was a