如果有别的更简单的方法能达到同样的目的，铁路工程师是不会选择开挖隧道的，这种说法一点没错。就单位长度而言，隧道是各种铁路工程中耗资最高的。开挖山区大型隧道存在的问题在工程领域几乎是无与伦比的。由于不可能在岩层里试掘几千英尺，因此当工程师身处地表以下很深的地方时，他无法确切地了解自己会遇到什么样的地质条件。这些条件可能包括地热、地下水、风化岩及许多别的危险。在掘进工作中，所有掘出的土石必须通过洞身狭长的通道运出来。工程师必须以最高的精密度来保证洞身走向。但是，隧道工程与地面线路施工不一样，工程师无法参照外部物体来检验它的位置。

    It is for sure that no railway engineer prefers to tunneling if there are other and simpler means of reaching his objective. Relatively to length, a tunnel is probable the most costly of all forms of railway engineering. The problems presented by the major tunnels in mountainous country are almost unique in the engineering realm, for the engineer, unable to make preliminary borings through perhaps thousands of feet of rock, has no certain knowledge as to the condition that he may meet when deep underground. These may include high temperature, underground streams, rotton rock and many other hazards. During the work of boring, all the excavated material must be brought out through the narrow confines of the bore itself, and in preserving the line of his bore, which he must do with the most minute accuracy, the engineer is deprived of any means of checking his position by reference to external objects, as he can do with any surface line.
    在松散、无粘着力或软土层进行隧道挖掘一般是使用钢制盾构。盾构在大多数情况下是圆形的，通风，但在必要时也可使用压缩空气。盾构的主体是一个钢盾圆形体，至少用两个圆形肋，并用垂直和水平的支撑件加固。这些支撑件把工作面分成许多小隔间以便用来开挖和出碴。隔间的大小要适合所使用的挖掘和出土的方法和设备。使用盾构可以使土壤挖掘和临时支撑都在安全的条件下进行，可以更好地控制地面下沉。而对于挖掘水下隧道而言，使用盾构更是必不可少的。自从证明带有铸铁内壁的盾构能够安全地建造水下隧道以来，这种盾构就得到广泛地使用，包括在通风情况下，或是处于压缩空气情况下，这类盾构用来在大城市建造地铁，大型下水道，后来又用于建造公路隧道。

    Tunnels in loose, non-cohesive, or soft ground are generally driven by means of steel shield, most often circular in shape, either in free air or, if necessary, under compressed air. The body of the shield is a steel cylindrical plate, stiffened by at least two circular ribs and vertical and horizontal bracing members. These divide the face into a number of pockets which are used for mining and mcking. The size of the pockets is adapted to the methods and equipment used for excavation and removal of soil. The shield permits the excavation of soil and the erection of primary lining under safe conditions; allows better control of ground settlement, and is a must in subaquesous tunnels.

    Once it was demonstrated that shields with cast iron lining could safely construct subaaqueous tunnels, such shields have come into extensive use, in free or in compressed air. These shield permits the building of large subway system in cities, large sewer and later, highway tunnels.

    正常条件下，较长隧道的挖掘是从两头同时开始，两边的施工队伍缓慢地前进，逐渐靠拢，经过一段时间，有时甚至需要数年的时间，两端的隧道才会相遇。这一时刻总是非常令人兴奋的。“为了确保两端隧道相遇，两者中心线的测定必须非常精确。例如，挖掘一条10英里的隧道即使开始时的误差仅为100英尺偏半英寸，那么，到了5英里处，偏差就会扩大到25英尺。在此情况下，两端隧道就根本不可能相遇了。所以，最初要通过三角测量在两个洞口之间的各个山头上，有时是通过弯弯曲曲的途径建立一套检测线系统。要做到准确挖掘，单是依靠即使是最可靠的地图也是不够的。通过这种三角测量，确定了隧道两端的入口线路，而且这两端线路必须一丝
不苟地保持其正确性，一直到两端地隧道相遇。

    In normal conditions boring of longer tunnels begins simultaneously at both ends, and the working parties advances slowly towards one another until, after a lapse, probably, of several years, the two working parties meet-- an occasion which is always one of great rejoicing. To ensure this meeting, it is obvious that the setting out of the two center-lines must be of the most extreme accuracy; in boring ten-mile tunne, for example, an error of no more than 0.125 in. in 100ft . of the vertical and horizontal line of either bore would have expanded to over 25ft . at the end of the fifth mile, in which event the two bores would not meet at all. So the first preliminary is to establish, by triangluation, a system of survey lines from one portal to the other, over the tops of the mountain passes and sometimes by quite a ciricuitous course. it would not be sufficiently accurate to depend on even the most reliable maps only for this purpose. From this triangluation the lines of entry to both bores are established, and these must be maintained with the most scrupulous exactitude until the meeting takes place.