operation throughout the northern coalfields. The known coal reserves of the northern coalfields were greatly increased, in 1886, when Professor T. W. Edgeworth David discovered the Greta coal seams, the coal resource which allowed a rapid expansion of coal mining on the South Maitland Coalfields. At the end of the 19th century, the four most important companies on the northern field were the Australian Agricultural Company, J & A Brown, Newcastle Wallsend, and Scottish Australian (Lambton Colliery). Of these, only the Scottish Australian was not a member of the Associated Northern Collieries. This was essentially a cartel that divided production as quotas for each of the participating colliery owners. There was a monetary mechanism under which collieries selling above their quota compensated those selling under their quota. To avoid companies just leaving coal in the ground, quotas were adjusted based on actual sales for the previous year. The arrangement was known as "the Vend" and operated for most of the years between 1872 and 1893, when it collapsed due to competition in the export market. Many collieries on the northern coalfield of NSW were named after collieries in the United Kingdom. Other names referenced the coal seam being mined and that confused the locality and identity of the mine further. With changes of ownership, mine names often changed and sometimes names associated with good-quality coal were moved to completely different collieries. Most mines of the northern coalfields were connected to the ports at Hexham and Carrington by an extensive network of railway lines; some were government lines and others were privately owned. Coal was loaded into four-wheel wagons owned by the mining companies. The type of four-wheel wagon used consisted of a frame with wheels and a removable wooden hopper. Trains of wagons were hauled to the port, where the removable hoppers were lifted out of the frame by a crane and dumped by opening the bottom of the hopper. The worker who opened the hopper was known as a 'pin boss', probably because he used a large hammer to knock out a locking pin that secured the hopper gate. The wagons were known as "non-air" wagons because they did not have air-brakes. At their peak, there were 13,000 of these "non-air" wagons in service, belonging to around sixty operators. Larger modern bogie wagons were introduced in the 1960s and "non-air" wagons were banned from Port Waratah in 1974 but