Railway tunnels may be built either to accommodate one or two, three and four tracks. In single-track tunnels a clear space of at least 2¹⁄₂ ft. on each side should be allowed for between the tunnel wall and the side of the largest standard locomotive or car, and a clear space of at least 3 ft. should be allowed for between the roof and the top of the same locomotive or car. Since the roof of the tunnel is arch-shaped, to secure a clearance of 3 ft. at every point will necessitate making the clearance at the center greater than this amount. In double-track tunnels the same amounts of side and roof clearances have to be provided for, and, in addition, there has to be a clearance of at least 2 ft. between trains. On the three- and four-track tunnels only the width varies while the height remains almost equal to the two track. Referring to Fig. 7, and assuming the line _AB_ to represent the level of the tracks, then the ordinary dimensions in feet required for both single- and double-track tunnels are as follows:-- +--------------+--------+----------+----------+---------------+ | | HEIGHT, | WIDTH, | HEIGHT, | HEIGHT, | | | D. F. | G. E. | C. F. | C. H. | | | FEET. | FEET. | FEET. | FEET. | +------------+----------+----------+----------+---------------+ |Single track|17.6 to 18|16.5 to 18|6 to 7.4|¹⁄₄ to ¹⁄₈ _AB_| |Double track|26.6 to 28|26.6 to 28|6.3 to 6.9|¹⁄₄ to ¹⁄₈ _AB_| +------------+----------+----------+----------+---------------+ The dimensions of tunnels built for aqueduct purposes are determined so as to have an area of cross-section equal to the required waterway. In the Croton Aqueduct two different types of cross-sections were used, the circular one for tunnels through rock and the horseshoe section for tunnels through loose materials. In the Catskill aqueduct three different cross-sections have been selected, the circular one for tunnels under pressure and the horseshoe for tunnels at the hydraulic gradient. These, however, through rock have a cross-section formed of a semi-circular arch and vertical side walls, while through earth the semi-circular arch is supported by skewback walls.