The best possible wood for sleepers is:

Explanation:

Wooden Sleepers :

• The natural properties of wood (usually hardwood such as oak, but cheaper softwood has been used on lighter, less busy lines) are suited to providing a resilient track with excellent dynamic attenuation of impact loading as well as noise and vibration reduction.
• There are many kinds of wood that can be used as railroad sleepers in the world, but not every kind of wood can be used.
•  At the different sections of the tracks, for example, the common track section, turnout track section, bridge track section, etc., we often have to select different kinds of wood to meet different conditions and environments.
• Meanwhile, wood can be divided into hardwood and softwood according to the hardness of the wood texture, which also needs us to make a proper selection for the specific tracks.
• Sal and teak both are hardwood. And most commonly used for sleepers in India

• Sal is both heavier and stronger than teak wood but because teak has great resistance towards fungal decay and white ants that's why it's considered best.

Additional Information

•  Shorea robusta is the botanical name of the Sal tree.
• The sal tree is indigenous to India and roughly one-eighth of the total forest area of India is covered by sal trees.
•  It is particularly found in the central and northeastern parts of the country up to the Himalayan foothills.
•  It is a species of tree in the family Dipterocarpaceae.
• This tree is native to the Indian subcontinent.
• It is a coarse-grained wood that is an important source of timber in India.

Sal wood is used in making railway sleepers, railway coaches, and furniture because:

•  It is very hard, tough, and heavy, which is suitable for bearing heavy weight.
•  Heartwood is naturally very durable.
•  It does not get affected by termites for a long period of time.
• As they are large trees, the wood is suitable for producing beams and poles too.