This small error in reckoning was easily rectified. Many tunnels are bored through solid rock by the aid of pneumatic and electric-drills and explosives. A temporary railway track is extended from the portal into the bore as fast as the drilling proceeds, and this is used to carry off loose rock and boring dust. It is also used to transport workmen, equipment and materials. Electric fans or ventilator pipes keep fresh air circulating in the bore.
Pumps draw off seepage water.
Some tunnels are built for one track only, others are built for two or more tracks, depending upon anticipated traffic and other operating conditions. Many tunnels are lined with concrete, brick or timber, or a combination of these materials, and many are made waterproof to prevent seepage. Linings strengthen the walls and ceiling and increase the safety of train operations. Many railroad tunnels are electrified, and trains are operated through them by electric locomotives.