LEGO Train - what size cork track bed
LEGO trains typically use a 2x8 plate for their sleepers (ties), with a stud protruding from each rail, giving a 40mm centreline gauge for the track and 64mm width for the sleepers - this is 5.5mm wider than O-Gauge - so might be a little hard to find off the shelf.

Lots more detail can be found on L-Gauge.org.