One T. F. Budden is credited with taking this photograph of the famous British steam train, the "Flying Scotsman".
And here's another illustration of what an old-fashioned steam locomotive looked like:
Shortly before diesel locomotives took over from the steam locomotive, some steam locomotives were given a streamlined design in order to make them look more modern and appealing.
One example of this is the locomotive used with the "Hiawatha" train:
Another is this locomotive, aptly called "The Bullet":
Here is a Hudson locomotive used by the New York Central railway:
perhaps closest to what many generally think of as the appearance of streamlined steam, from a photograph generously donated to the Library of Congress and placed in the public domain by Gottscho-Schleisner Incorporated.
This is a class S-1 locomotive from the Pennsylvania Railroad, as placed on display at the 1939 New York World's Fair;
from a photograph generously donated to the Library of Congress and placed in the public domain by Gottscho-Schleisner Incorporated.
And here is a streamlined steam locomotive belonging to the B&O Railroad. Which, in the world of railroads, stands for the Baltimore and Ohio Railroad; not for Bang & Olufsen, as it does in the world of high fidelity.
The T-1 streamlined steam locomotive from Baldwin had a rather different appearance:
Diesel locomotives replaced steam locomotives because they had lower operating costs. The danger of the boiler of a steam locomotive exploding meant that these locomotives required frequent inspections and maintenance. As well, along the way, they had to be frequently supplied with water.
One of the earliest diesel locomotives had an engine made by the Winton subsidiary of the Electro-Motive Division of General Motors, put in a streamlined body designed by Pullman, the famous maker of passenger cars:
This M10000 was commissioned by Union Pacific, and put into service as the City of Salina.
And here is a streamlined diesel locomotive from Baldwin:
The type of diesel locomotive many of us remember from our childhoods looked like this:
or like this:
Today, though, the diesel locomotives you are likely to see pulling trains will look more like this:
This design, while less appealing to the eye, offered a number of practical advantages. It was easier to get at the diesel engine inside the locomotive for maintenance. It was possible to stand on the train, but outside of it, so as to have a good view of the train's coupler when it was being coupled to another car in front of it.