Why are there two locomotives?

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TylerP42

TylerP42

Conductor
Joined
May 22, 2015
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1,223
Location
Detroit, Michigan
May be a dumb question, but my friend and I were talking about it.

Why do long distances trains have 2 locomotives?

8,500HP for 12 car trains, little ridiculous if you ask me.

I understand on a western trip where if something happens to one and there isn't much help in sight, but on trains like the Capitol Limited and Lake Shore Limited it's not really necessary. I mean if something happens to one, they usually remove it and put a freight engine on the front.
 
I suspect that NS *really* doesn't like it if a train loses power on the extremely busy Chicago Line (Chicago-Cleveland), as it can cause traffic snarls extending halfway across the country.

Apparently CSX is not quite as insistent about having two locomotives on the Florida trains, as this has happened relatively often.
 
Part of the reason is due to the HEP. One P42 can power up to 16 cars and with that the traction horsepower is greatly reduced due to the power being sent back but also the alternator runs at a slower RPM in that mode. Shorter trains (corridor service, TE...) can run with just one locomotive as there is enough Power to split between the two.
 
Waiting for a unit from the Host RR can take a while plus now two engineers are required, Amtrak and Host RR. Besides the hours of delay for Amtrak, the host RR has a blockage on a busy freight corridor. The contract between Amtrak and the Host RR could include Amtrak running two units on certain track.
 
The Coast Starlight was running with one freight engine and one Amtrak engine in December. I asked the Amtrak engineer if a freight engineer was required; he said no, just the Amtrak engineer.
 
In most cases it boils down to a short list of reasons: 1. HEP requirements 2: Host railroad requirement Length/weight of train and grades on route (as an example CZ East of Denver is mostly flat, West of Denver needs 2 units)
 
Also if the host railroad has to supply freight locomotives, they are limited to 70 MPH instead of 79.
 
Not to mention some lines have rules for passenger train length. On the Hamlet Sub of CSX. A passenger train of over 12 cars must have two locomotives both on line. And all of us should know along the Silver Star that two locomotives when it comes to grade crossing incidents are a nice thing to have
 
swc34 said:
Part of the reason is due to the HEP. One P42 can power up to 16 cars and with that the traction horsepower is greatly reduced due to the power being sent back but also the alternator runs at a slower RPM in that mode. Shorter trains (corridor service, TE...) can run with just one locomotive as there is enough Power to split between the two.
Click to expand...
This ia really good info. I had read once that the HEP is split between the two engines. I never knew that a P-42 could handle 16 cars. This sort of info fascinates me and I love reading specs and details. Thanks for posting this!!
 
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The way that Amtrak cars are wired does not allow for both engines to provide power. The engines would have to be synchronized with each other, and that is harder than people think. My father works for the local utility in generation, and when a generator is brought online the frequency needs to match the rest of the grid before the generator is allowed to produce electricity, or it would cause wave interference on the grid.
 
If "separately derived systems" are providing power simultaneously, they must be phase synched The "wave interference" the previous poster mentioned can be as serious as a big explosion. Crossing phases is a major electrical "kaboom" If you had 2 smaller emergency generators supplying a facility, you would have equipment known as "paralleling gear" installed specifically for that purpose before the transfer switches.
 
Paralleling generators has come a long way.

1. We are all aware of the National grid is 3 phase parallel

2. When the first AC powered aircraft ( B-50 ) power buses were all separate.

3. then when the KC-135 was built it had provisions to parallel. Its derivative the B-707 , & B-727 followed. All done manually with neon indicator lights

4. Then the DC-8 came out with auto paralleling and the Lockheed L-1011.

5. For various reasons ( just 2 generators for one reason ) DC-9s , B-737, B757, b767 B777 are all separate buses that can connect only is one generator fails.

6. Other aircraft unsure.

A problem with parallel generator is that one will always be very slightly faster than another but load division will tend only to put slightly ( 2 - 5 % ) more load on faster generator.

Suspect that Amtrak can get more total traction power by using just one prime mover. Plus if a sudden short takes out HEP system other loco(s) can be switched in.

There is some confusion of what the HEP available of various locos are even the same model. We do know that Amtrak changed its auto train P-40s to a higher KW.

The ultimate of course is the ACS-64s which have two separate 1000KW HEP systems but they do not parallel. But if one fails the other can be used. They do not affect total traction power of 6400 KW.

As for new diesels HEP output unknown.
 
When the P-40s were rebuilt to raise the max output of the prime mover, did they change the HEP setup, or just send the added power to traction? Another wrinkle are the "genset' locomotives, where the HEP is generated by a separate engine leaving traction power unaffected. The technology involved in combined/parallel electrical systems large or small (especially control and monitoring) has advanced tremendously, the physics that requires it remains the same.
 
Also keep in mind that most Amtrak trains running with 2 units are actually only using 1.

Amtrak rules dictate that one unit be offline to conserve fuel unless you need both units online to make track speed. Usually the 2nd unit is just there as a backu0 and if everything goes right it's never even used.
 
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Until a few years ago, the Builder three units were standard power for years one of the only trains on the system with that feature.
 
The p42's have perennial problems in the winter, usually with their traction motors and snow, so Amtrak would assign three to the builder to keep it moving even if one fails. Since it is the only western LD that has to operate through snow for all but maybe fourth miles of its run. This makes it the most likely to have a failure.
 
It also has to do with fleet balancing. Some trains actually need the second unit for portions of the trip to meet the schedule. So, if you constantly send out "x" amount of units, your facilities need to receive "x" amount of units.
 
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