We have all built some sort of engine terminal on our garden
layouts.  In most cases it is a small engine house, a water tower and
maybe a coaling facility.   There are some other items that can be added to
this scene to make it more believable.
     Steam locomotives of any size required a great deal of
maintenance.  The small engine house is perfect, since most steam loco
repairs required that the locomotive be in out of the weather and
protected.   Diesels also use engine houses, but most 1st generation diesel facilities
occupy buildings that were originally used for their steam predecessors.

      The most common fuel for steam locomotives was coal.  Moving coal
to the tender required some sort of structure above the tracks that used
gravity to load coal into the tender.  This structure was called a
tipple, and was constructed of wood or reinforced concrete.  A spur was
run behind the tipple, and hopper cars filled with coal were unloaded
and the coal hoisted to the top of the tipple using some sort of conveyor.
      Oil burning locomotives required a tank to store fuel, a pump
house, and standpipe to pump fuel into the tender.  In the early days,
this fuel was "Bunker C" which sometimes required heating to get it to
flow properly, especially in cold weather.  More modern refined fuels did
not require this extra heating step.  As a result, your engine facility may
require one or both of these fueling areas depending upon the type of steamer
you are running on your railroad.
       Another "must" for steam locos is the use of sand.  One of the
domes on your steam loco contained sand for driver traction when
required.  This sand had to be dry to allow it to flow freely.  Sand
facilities had equipment to dry the sand before it was conveyed to a
hopper above the tracks for loading into locomotives.  Sand bins were
sometimes a part of the coal tipple, and sometimes they stood alone.
They were either supplied with sand by hopper, or via truck.
       Water of course is an essential for steam locomotive operation.
Before heading out for a road assignment. the were fueled, sanded, and
finally filled with water.   This requires that you provide a water
tower or standpipe and pump house.
Steam locomotives required a great deal of lubrication.  55
gallon drums on stands with spigots were always found around the engine
terminal.  In large facilities, a building often was used to store
lubricating materials.
       One item not included in many layouts is an ash pit for your
steam locos.  When a coal fired steam locomotive was brought in after a run, the
fire was actually dropped before the loco went into the engine house.
This pit was below the track and had to be emptied periodically when it
started to fill up.  A lot of times this ash was used to pave yard areas
and the parking lot in an engine facility.
        As I mentioned earlier in this article, diesel facilities were
often steam locomotive facilities converted to a new use.  During the
transition period, diesel and steam locomotives shared the same
facilities.  As a result, you often see photos of diesels running past
coal tipples as the exit the yard.  Diesels would often use the same
sand facilities that the steam locomotives used.
        Diesel fueling is done with hoses connected to standpipes.  A
pump house would be required to pump the fuel to the standpipe  While
some railroads built tanks to store their diesel fuel, some used tank
cars on a nearby siding for fuel storage.  Diesels also require sand,
and in many cases the same sanding facilities are used that were built
for steam locomotives.  Diesels use water for cooling, but this was
often provided by a hose.  Diesels also require lubricants and engine
oil, so a storage building will be needed.
        Other items included in the engine terminal would be equipment
for fire fighting.  This can represented by small shacks painted red
which usually contained hoses connected to a centralized water pumping
system.  There would also be many fire extinguishers located around the
engine facility.  Buildings would be needed to store materials such as
paint, waste for cleaning, drinking water, etc.   A wash rack was often
found in the terminal to wash the locomotives down after a run.  This
was especially true of passenger locomotives.
        Finally, there needs to be buildings to house the shops where
machine tools and parts were kept, along with a yard office.  If you
don't have a building for this purpose, an old boxcar with windows and
doors added will work.
        All of these items have the ability to add detail and interest to your
locomotive facilities.  Look at some pictures of the real things to get a
better idea of what these structures looked like and where they could be
placed in your yard area.  Many of these items are available from Piko or Pola,
but many can be easily scratch-built if you are so inclined.