When we look at the prototype rails, they are sat on a thick bed of ballast. Track ballast is the material which forms the track bed upon which railway sleepers are laid. It is packed between, below, and around the sleepers, and is used to bear the compression load of the sleepers, rails, and rolling stock. The stones used are chosen to facilitate drainage and keep down vegetation that can compromise the integrity of the combined track structure. The ballast also physically holds the track in place as the trains roll over it. (https://en.wikipedia.org/wiki/Track_ballast)
For our model railways, there are as many opinions as there are options, what you choose very much depends on your personal choice.
Assuming you want to make a semi-permanent layout, the simplest option is 'nothing'. You can lay your track directly on the baseboard, and either glue or pin it down. Model ballast can then be spread between and around the sleepers and fixed with glue.
For a basic representation of ballast, most track manufacturers also supply pre-moulded foam underlay. This is available in a roll to fit the plain track, with specialist pieces designed to fit the different items of point work. Lightly pinned or glued to the baseboard, this also provides some sound-deadening capabilities. Visually, this gives the track a raised profile and a 'shoulder' of ballast. It will also help to minimise any lumps or ridges in the baseboard surface, providing a smoother passage for the trains.
Cork sheet is a common option. This can be obtained in several thicknesses, and of course can be laid with more than one layer if required. The cork can be laid across the whole baseboard or cut to fit the track outline. Cork floor tiles can also be used.
Cork strips, or roll, is a more specialised, made-for-model railway product. Like the foam underlay, it is just laid where the track will be placed. For a more advanced effect, cork strip can also be used on the outside of long curves to provide the 'cant or superelevation' effect.
A newer option is a denser sponge material, such as 'Track-Bed (TM)' from Woodland Scenics. This is available in strip, roll or sheet formats, allowing you to cut and match the underlay to your chosen track layout.
Whatever you choose to go under your track, one idea is to paint the surface prior to adding track or ballast. Using a colour as like the intended ballast will help provide coverage if the ballast becomes loose over time.
A quick summary:
|What to use
|Can amplify rolling stock noise
|Can reduce noise
|Expensive, can deteriorate over time
|Simple, easily to obtain, can provide ballast shoulder
|Needs glueing and cutting to shape
|Moderate price, provides ballast shoulder
|Can be difficult to curve
|Reduces sound, helps overcome baseboard irregularities
|Expensive for a larger layout