The line is presently single track. To the north, at Berryden Junction, it has been redoubled through to Inverurie (2019) and to the south, at Aberdeen, the line is double. The tunnel was built to accommodate a double track. It is 250 yards long.
eBay has some items listed which may be of interest. (Note that eBay sellers sometimes mis-identify locations.) RailScot is an eBay affiliate. Buying an item may help cover the running costs of the site.