The General Motors model F40PH is a 3000 horsepower B-B diesel-electric locomotive intended for service on Amtrak's intercity lines, built in 1976. This locomotive is equipped with a turbocharged 645E3 16 cylinder, 2 stroke water cooled "Vee" diesel engine (prime mover) that develops 3000 horsepower at maximum RPM. The main (traction) generator converts mechanical energy from the prime mover into electrical energy that is distributed through a high voltage cabinet to the traction motors. Each of the four traction motors are directly geared to a pair of driving wheels. The gear ratio of the traction motors to wheel axle determines the maxinum operating speed of the locomotive; a standard F40PH has a gear ratio of 57:20 which provides a top speed of 103mph.

The F40PH has a fully enclosed carbody which provides protected walkways for easy access to the engine room and trailing units. This arrangement allows routine maintence while the locomotive is in service.

To support passenger service, the F40PH is equipped with a secondary electrical generator referred to as the head end generator. The head end generator generates AC power at 480vac 750 amps or about 500kW to provide power to the passenger cars for lighting, electric heating, and air conditioning. Because head and power must be delivered to the rest of the trainset at a relatively constant frequency, the prime mover in these locomotives must turn at a constant high speed (900 RPM) while delivering head end power (even if the locomotive is standing still at a station stop); this has led to these locomotives being nicknamed "screamers". Power to the traction motors is varied by varying the field excitation of the main (traction) generator.

Later variations on the F40PH have added a second diesel engine that exclusively drives the head end power generator. In these versions of the locomotive, the prime mover can vary its speed depending on the amount of tractive effort needed. These locomotives can be identified by a small secondary diesel exhaust at the rear end of the locomotive (besides the fact that they can idle quietly).

Diesel locomotives built by GM-EMD
Switchers:	SC, NC, NC1, NC2, T, NW2, NW3, NW4, NW5, TR1, SW1, SW7, SW8, SW9, SW600, SW900, SW1000, SW1001, SW1200, SW1500, SW1504, MP15DC, MP15AC, MP15T, GMD1, RS1325
Four axle road power:	BL2, GP7, GP9, GP15-1, GP15AC, GP15T, GP18, GP20, GP20D, GP28, GP30, GP35, GP38, GP38AC, GP38-2, GP39, GP39DC, GP39X, GP39-2, GP40, GP40P, GP40P-2, GP40TC, GP40X, GP40-2, GP49, GP50, GP59, GP60, GP60B, GP60M
Six axle road power:	SD7, SD9, SD18, SD24, SD28, SD35, SDP35, SD38, SD38AC, SD38-2, SD39, SDL39, SD40, SD40A, SD40-2, SD40-2W, SD40T-2, SD402-SS, SDP40, SD45, SD45X, SD45-2, SD45T-2, SDP45, SD50, SD50F, SD50S, SD60, SD60F, SD60M, SD60MAC, SD70, SD70ACe, SD70M, SD70M-2, SD70MAC, SD80MAC, SD90MAC
Eight axle road power:	DD35, DD35A, DDA40X
Cab units (F- & E-units)	FT, F2, F3, F7, FP7, F9, FP9, FL9, TA, EA/EB, E1, E2, E3, E4, E5, E6, E7, E8, E9, AB6
Cowl units:	F45, FP45, F40C, F40PH, F40PH-2, F40PH-2C, F40PH-2M, SDP40F, SD40-2F, SD50F, F59PH, F59PHI, SD60F
Electrics:	SW1200MG, AEM7, GM6, GM10, GF6C
Industrial, military, export and narrow gauge:	Model 40, DHI, GMDH-1, GMDH-3, MRS-1, GA8, G8, G12, G16, NF210
Streamliners:	M-10000, M-10001, M-10002, M-10003-6, Pioneer Zephyr, General Pershing Zephyr, Green Diamond, Flying Yankee
Experimentals:	EMD 1800 hp B-B