Specific maintenance operations in tunnels are always made difficult by cramped working space. Out-of-
face rail renewals, which usually employ rail-mounted cranes, are complicated by the difficulty of operating them
in such close quarters. Ties are similarly difficult to replace in tunnels.
The many difficulties associated with track maintenance in tunnels necessitate taking every possible step
to reduce such work to a minimum. For this reason, many railroads use welded rail in tunnels to eliminate joints
and their constant maintenance problems. Some railroads have substituted concrete ties in tunnels to avoid the
frequent replacement of timber ones. When tunnel maintenance is required, using out-of-face methods is a better
policy than spot renewals or repairs. The entire job is completed at once; frequent small jobs that must be done
under poor conditions are avoided.
Track officials usually classify bridges as either ballasted-deck or open-floor. A ballasted-deck bridge
has a solid flooring on which a conventional roadbed is placed, as shown in figure 5.1. On an open-floor bridge,
the ties are fastened directly to the bridge structure without ballast, as you can see in figure 5.2A. This type is less
satisfactory than the ballasted-deck bridge from the standpoint of track maintenance. However, an open-floor
bridge is cheaper to build and easily converted to a highway bridge, and, for this reason, is more often constructed
in a theater of operations. The top view of an open-floor bridge shown in figure 5.2B gives an idea of how easily
this conversion could be made.
Figure 5.1. Ballasted-Deck Bridge.