From: bill@mathnx.math.byu.edu (Bill Smith)
Newsgroups: rec.railroad
Subject: Re: ? about engine sets
Date: 22 Jan 1993 17:56:51 GMT
Organization: Brigham Young University
Distribution: usa
NNTP-Posting-Host: mathnx.math.byu.edu

In article <1jp1qtINN4as@grapevine.EBay.Sun.COM> maral@repare.EBay.Sun.COM  
(Mark Maral) writes:
> My son asked me a question the other day and I couldn't answer him
> so I thought I would tap the knowledge of all you experts.  The
> question is why do trains utilize various engine types when pulling
> a string of cars and why do some point forwards and some backwards?
> I kind of would like to know myself.  A 10 year old and 36 year old kid
> appreciates your response.
> 
> 

Well, as far as number and type of locomotive, the idea is to try to match
available horsepower with train tonnage/terrain/time (maybe weather and load  
type). For example, the railroad has to run  X tons  up Y% grade in Z hours.   
Available motive power at the beginning of the trip is matched with the  
horsepower requirement to make the trip in required time.  

Of course its more complicated than this.
Some locomotives may be assigned exclusively to certain service (maybe
U.P. uses SD60s on certain coal trains say)
The power needed at the other end of the trip may effect the number and
type of locomotives used (we will need  10,000 h.p. at the destination location  
so tack on extra units to this train).  Other factors may enter as well like
service time - need for maintance of the locomotives - hence some units might  
be routed toward a major service facility.  Also helper
service may change the configuration over some parts of a road.
Other items like track type/condition may dictate type of locomotive. 
Some railroads might have rules about how locomotives are to be 
arranged when used in multiple unit configs.  Lead unit generally
has short cab first on most railroads.  Orientation of trailing units
might depend on whether helpers are used and what happens at the other
end of the journey as well as railroad rules about last unit having
long hood first perhaps.

Some railroads have their custom computer code which keeps track of all
this stuff and more on any given run.  Others might choose to use less
than optimal power (maybe they don't have it, or helper service is too
expensive) because moving the load is more important than burning
up a traction motor. (D&RGW used to be famous for this)

-Bill 
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