
Steam
Locomotion in the 21st
Century
The Recent
History of Steam Locomotive
Development

Philosophy of Modern
Steam Locomotive Design
updated 4 February 2022
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Much thought has been given to the best
form and characteristics for steam
locomotives. Different countries and
different designers had different ideas
about this, some very valid and some with
no basis in science or engineering! These
philosophies were applied with varying
degrees of success. More recently, with
steam projects in the 1980's, alternative
design philosophies were adopted to
attempt to gain interest from commercial
railways (i.e. maximum compatibility with
existing diesel operations). The following
lists concentrate on the basic design
ideas that should be applied to classical
Stephensonian steam locomotive designs.
In the U.S., the Norfolk and Western
Railway set forth a definition for a
"modern steam locomotive" in the 1940's
which still provides valid criteria for
steam locomotives today. These criteria
are typical of the final generation of
steam locomotives built in the U.S. for
mainline railway service.
Norfolk
& Western Railway
definition of a
"Modern Steam Locomotive" 
1.
A highcapacity boiler
2. Roller bearings on all engine
and tender wheels
3. A onepiece cast steel bed
frame
4. Improved counterbalancing
5. Complete pressure and
mechanical lubrication 
While this definition covers
many important points, it is not complete
and significant important design features,
that would be desirable in a truly modern
steam locomotive, are missing. It also
reflects the U.S. emphasis on achieving
power through high steam output
(highcapacity boiler) as opposed to the
alternative idea of maximizing power by
minimizing steam consumption. Famous
French steam locomotive designer Andre
Chapelon incorporated other important
design characteristics in his designs:
Andre
Chapelon 
 A highefficiency exhaust
system.
 High degree of superheat.
 Streamlining of the Steam
Circuit.
 Compounding

Chapelon's ideas point to the maximizing
of locomotive efficiency. While Chapelon
is best known for his work improving steam
locomotive thermal efficiency, he
extensively studied U.S. steam locomotive
designs and was incoporating the ideas
stated in the N&W "Modern Steam
Locomotive" definition in his planned
postWWII locomotives.
Following Chapelon, L. D. Porta's
locomotive designs have incorporated the
following additional features which would
be desirable for any new steam locomotive:
L. D.
Porta 
 Efficient combustion
system
 Maximization of adhesion
 Advanced tribology
 Careful attention to
detailed design

In his book The Red Devil and Other
Tales from the Age of Steam, David
Wardale listed the following criteria,
which incorporate all of the above ideas
plus additional ideas of his own.
Wardale's criteria capture many ideas
previously incorporated in various
designs, but never put to paper as a list
of design criteria:
David
Wardale 
 Maximize the boiler
pressure.
 Minimize the boiler 
steam chest pressure drop:
requires the largest flow
area throughout the live
steam circuit, particularly
through the superheater
elements.
 Minimize the steam chest 
cylinder pressure drop;
requires the largest steam
chest, the largest diameter
valves of longest lap and
greatest flow coefficient,
and valve liners with the
largest ports of highest
flow coefficient.
 Minimize the exhaust steam
back pressure: requires the
largest exhaust passages of
highest flow coefficient,
the best possible exhaust
system and the lowest
possible boiler gas flow
resistance compatible with
efficient boiler operation
and high superheat.
 Ensure that the boiler
pressure / cylinder volume /
coupled wheel diameter allow
high power to be developed
in the normal speed range at
economical cutoffs.
 Minimize the cylinder
clearance volume.
 Maximize the steam
temperature: requires the
largest superheater and the
highest fraction of the
total combustion gas
sweeping the elements.
 Maximize the feedwater
temperature: requires the
largest feedwater heater of
maximum heat transfer
coefficient.
 Minimize the boiler
unburnt fuel loss: requires
the minimum burning rate,
achieved by maximizing the
cylinder and drawbar
efficiencies and hence
minimizing the boiler energy
output rate for the required
power, and the best
combustion system.
 Minimize the combustion
excess air: requires the
best fuel / air mixing in
the combustion zone.
 Ensure that the draughting
and combustion systems
guarantee good steaming so
that the maximum boiler
pressure can be maintained
in practice.
 Maximize the locomotive's
power : weight ratio:
requires the maximum boiler
energy output rate and the
highest cylinder efficiency,
the smallest boiler, the
lightest superstructure, the
minimum number of carrying
wheels, and the lightest
tender consistent with
operating requirements.

The overriding design principle embodied
in Wardale's list is that each individual
aspect of the locomotive must be designed
as well as is possible, a
philosophy which was sadly absent in many
previous steam locomotive designs.
