being single track that were later duplicated. The Bendigo and Geelong–Ballarat mainlines were both built as double track in the 1860s, but were singled in the 1930s and first decade of the 21st century respectively. The north-east line to Seymour was duplicated in the mid-1880s and remains so today, the Gippsland line to Moe was duplicated in the 1950s in conjunction with increased briquette traffic, and the busiest country line in the state to Geelong was progressively duplicated from 1959 to 1981. Few railway tunnels exist in Victoria, with the exception of the Melbourne City Loop. The longest tunnel before the opening of the loop was on the Fyansford Cement Works Railway (near Geelong), where a 1300-metre-long tunnel existed on a narrow gauge quarry railway. Of those open today the longest is the single track 422-metre long Geelong Tunnel, followed by the double track 385-metre Elphinstone and the 390-metre-long Big Hill Tunnels on the Bendigo line. A 154-metre-long tunnel also exists on the Healesville line, as well as three tunnels on the suburban Hurstbridge line, and another on the freight lines under Footscray station. On the interstate railway corridors, there are independent sections of track due to the use of both broad and standard gauges in the state, these sections being from Melbourne to Seymour and on to Albury (constructed in the 1960s), and Melbourne to Geelong (completed in 1995). The gauge issue also sees dual gauge track used, in areas including Maryborough, North Geelong, and various freight terminals in Melbourne. The maximum speed of suburban electric multiple units and locomotive hauled trains is 115 km/h, with diesel multiple units permitted up to 130 km/h on the same track, and up to 160 km/h on specified lines. The maximum axle loading of freight wagons is 20 tonnes, with locomotives of up to 22 tonne axle loading operating. Train lengths are limited to 1200 metres, except on the main interstate lines where 1500 metre long trains are permitted. Track gauge The majority of the railways of Victoria are of broad gauge, presenting break-of-gauge difficulties when connections were made with New South Wales which uses standard gauge. In addition, the Victorian Railways experimented with four short narrow gauge lines of in the early 20th century. Efforts to eliminate the gauge issue were proposed many times in the intervening years, with a Royal Commission in 1921 deciding "that the gauge of 4-ft. 8.5-in.