True English oak (Quercus robur) rather than one of the less durable, imported Quercus species like Turkey oak.
Oak sleepers are cut from a lower grade of oak than products such as oak planks and oak beams. This means that oak sleepers contain more defects (or character!), which might include large knots, sapwood, wane and splits.
Softwood Sleepers
Creosoted Railway Sleepers