The inner workings of Head End Power (HEP)

The Abraham Lincoln lost its Amtrak certification when new regulations required all cars in the Amtrak train to be equipped with Head End Power (HEP), but what exactly is HEP?

In the early days of the railroad, passenger cars in the train were heated with steam from the locomotive and generated their own electricity from a wheel driven generator on each car. Besides the coupling, the only connection between the cars were the air hoses used to control the simultaneous application of the air brakes on each car. An air line along the length of each car, and a hose at each end, passed the air (and the braking signals contained in the air pressure) to the next car and so on down the line. Actually a very sophisticated system, the brake system in each car used these pressure signals to control the air pressure apply its brakes. This system is still in use and can be seen as the air hoses sagging between each car on a freight train. For freight trains, the crew members in the caboose (when cabooses were used) would monitor this air pressure looking for problems with the system and watching the signals transmitted from the engineer in the locomotive. As a note, this is why the Abraham Lincoln is fitted with air pressure gauges in the observation room.

While the air line is still used today to control the individual air-brakes of each car, head end power is the electrical equivalent of the locomotive-to-caboose air line. This electric line provides the "hotel load" power to every car in the train which is a substantial electric load. For each individual car in the train, this power (generated by the locomotive or a generator car) provides electricity for the lights, air condition, heating, receptacles, etc. For the railroad, the major benefit of HEP is that cars do not need to generate their own power (simplifying equipment) and cars have power even when they are not moving. Amtrak also has standards to pass communication cables along the length of the train from car to car.

Since the Abraham Lincoln had the required air brakes, as long as it was the final car on the train, the lack of head in power was not an issue. The car generated its own electrical power (from an onboard diesel generator), and there were no additional cars behind the private car to require power. While getting electricity from the locomotive is nice, for many cars it did not justify the addition of all the equipment required to be electrically connected with the rest of the train. The Abraham Lincoln was in that category; it would be nice...but...oh the cost.

However, Amtrak has changed their regulations so that for any car to be certified for use with Amtrak, regardless of its position in the train, it must be equipped with head end power to pass electricty along to the rest of the cars in the train. Since this entails the transfer 1.2 megawatts of power (four cables, each carrying 400 amps at 480 volts, 3-phase), as one can imagine, this is not inexpensive equipment. To add Amtrak certified HEP to the Abraham Lincoln will require the addition of special receptacles at each end, special cables down the length of the car, and special power distribution equipment mounted aboard the car.

To obtain this certification, each car must be designed to carry the full electrical load of the train since, theoretically, any car could be the car immediately following the locomotive or generator car that is supplying the electrical power. Each car must have compatible electrical plugs and sockets at each end and these plugs and sockets scheme is designed to both transmit power while protecting passengers. To accomplish this, each car has a plug and receptacle on each side of each end (i.e., each corner) for a total of four plugs and four receptacles. When connected together, the plug and receptacle on each corner is connect to the plug and receptacle of the adjacent car. In this manner, the load is actually split among the four sets of cables and the car is symmetric with respect to the connections, meaning it doesn't matter which end if facing the locomotive or generator. Each plug and receptacle has three power leads and three smaller leads for ground and sense wires. The last car in the string has its plug on one side connected to the receptacle on the other and vice-versa. In this manner, there is a continuous connection of the sense wire from one side of the locomotive down the entire train to the last car where it crosses over and goes back to the locomotive. If this sense wire is broken, meaning that one of the plugs is disconnected or possibly worse, then the locomotive will remove power from the entire system, and all four lines from front to back go dead.

Amtrak's Heritage Fleet program was started was started in 1977 to equip older cars from Amtrak's predecessor railroads to Head End Power. These conversions were performed at Amtrak's heavy repair center in Beech Grove, Indiana, outside of Indianapolis. The program was completed by the mid 1980's.

For those individuals really interested in HEP and looking to maybe buy and send us a part or two, here is a link to a HEP equipment provider. We can provide you with a shopping list should you be so inclined.

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